Allergies

Definition

Allergies are abnormal reactions of the immune system that occur in response to otherwise harmless substances.

Description

Allergies are among the most common of medical disorders. It is estimated that 60 million Americans, or more than one in every five people, suffer from some form of allergy, with similar proportions throughout much of the rest of the world. Allergy is the single largest reason for school absence and is a major source of lost productivity in the workplace.

An allergy is a type of immune reaction. Normally, the immune system responds to foreign microorganisms or particles by producing specific proteins called antibodies. These antibodies are capable of binding to identifying molecules, or antigens, on the foreign particle. This reaction between antibody and antigen sets off a series of chemical reactions designed to protect the body from infection. Sometimes, this same series of reactions is triggered by harmless, everyday substances such as pollen, dust, and animal danders. When this occurs, an allergy develops against the offending substance (an allergen.)

Mast cells, one of the major players in allergic reactions, capture and display a particular type of antibody, called immunoglobulin type E (IgE) that binds to allergens. Inside mast cells are small chemical-filled packets called granules. Granules contain a variety of potent chemicals, including histamine.

Immunologists separate allergic reactions into two main types: immediate hypersensitivity reactions, which are predominantly mast cell-mediated and occur within minutes of contact with allergen; and delayed hypersensitivity reactions, mediated by T cells (a type of white blood cells) and occurring hours to days after exposure.

Inhaled or ingested allergens usually cause immediate hypersensitivity reactions. Allergens bind to IgE antibodies on the surface of mast cells, which spill the contents of their granules out onto neighboring cells, including blood vessels and nerve cells. Histamine binds to the surfaces of these other cells through special proteins called histamine receptors. Interaction of histamine with receptors on blood vessels causes increased leakiness, leading to the fluid collection, swelling and increased redness. Histamine also stimulates pain receptors, making tissue more sensitive and irritable. Symptoms last from one to several hours following contact.

In the upper airways and eyes, immediate hyper-sensitivity reactions cause the runny nose and itchy, bloodshot eyes typical of allergic rhinitis. In the gastrointestinal tract, these reactions lead to swelling and irritation of the intestinal lining, which causes the cramping and diarrhea typical of food allergy. Allergens that enter the circulation may cause hives, angioedema, anaphylaxis, or atopic dermatitis.

Allergens on the skin usually cause delayed hypersensitivity reaction. Roving T cells contact the allergen, setting in motion a more prolonged immune response. This type of allergic response may develop over several days following contact with the allergen, and symptoms may persist for a week or more.

Causes and symptoms

Allergens enter the body through four main routes: the airways, the skin, the gastrointestinal tract, and the circulatory system.

• Airborne allergens cause the sneezing, runny nose, and itchy, bloodshot eyes of hay fever (allergic rhinitis ). Airborne allergens can also affect the lining of the lungs, causing asthma, or conjunctivitis (pink eye). Exposure to cockroach allergens has been associated with the development of asthma. Airborne allergens from household pets are another common source of environmental exposure.
• Allergens in food can cause itching and swelling of the lips and throat, cramps, and diarrhea. When absorbed into the bloodstream, they may cause hives (urticaria) or more severe reactions involving recurrent, non-inflammatory swelling of the skin, mucous membranes, organs, and brain (angioedema). Some food allergens may cause anaphylaxis, a potentially life-threatening condition marked by tissue swelling, airway constriction, and drop in blood pressure. Allergies to foods such as cow's milk, eggs, nuts, fish, and legumes (peanuts and soybeans) are common. Allergies to fruits and vegetables may also occur.
• In contact with the skin, allergens can cause reddening, itching, and blistering, called contact dermatitis. Skin reactions can also occur from allergens introduced through the airways or gastrointestinal tract. This type of reaction is known as atopic dermatitis. Dermatitis may arise from an allergic response (such as from poison ivy), or exposure to an irritant causing nonimmune damage to skin cells (such as soap, cold, and chemical agents).
• Injection of allergens, from insect bites and stings or drug administration, can introduce allergens directly into the circulation, where they may cause system-wide responses (including anaphylaxis), as well as the local ones of swelling and irritation at the injection site.

KEY TERMS

Allergen— A substance that provokes an allergic response.

Allergic rhinitis— Inflammation of the mucous membranes of the nose and eyes in response to an allergen.

Anaphylaxis— Increased sensitivity caused by previous exposure to an allergen that can result in blood vessel dilation and smooth muscle contraction. Anaphylaxis can result in sharp blood pressure drops and difficulty breathing.

Angioedema— Severe non-inflammatory swelling of the skin, organs, and brain that can also be accompanied by fever and muscle pain.

Antibody— A specific protein produced by the immune system in response to a specific foreign protein or particle called an antigen.

Antigen— A foreign protein to which the body reacts by making antibodies.

Asthma— A lung condition in which the airways become narrow due to smooth muscle contraction, causing wheezing, coughing, and shortness of breath.

Atopic dermatitis— Infection of the skin as a result of exposure to airborne or food allergens.

Conjunctivitis— Inflammation of the thin lining of the eye called the conjunctiva.

Contact dermatitis— Inflammation of the skin as a result of contact with a substance.

Delayed hypersensitivity reactions— Allergic reactions mediated by T cells that occur hours to days after exposure.

Granules— Small packets of reactive chemicals stored within cells.

Histamine— A chemical released by mast cells that activates pain receptors and causes cells to become leaky.

Immune hypersensitivity reaction— Allergic reactions that are mediated by mast cells and occur within minutes of allergen contact.

Mast cells— A type of immune system cell that is found in the lining of the nasal passages and eyelids, displays a type of antibody called immunoglobulin type E (IgE) on its cell surface, and participates in the allergic response by releasing histamine from intracellular granules.

T cells— Immune system cells or more specifically, white blood cells, that stimulate cells to create and release antibodies.

People with allergies are not equally sensitive to all allergens. Some may have severe allergic rhinitis but no food allergies, for instance, or be extremely sensitive to nuts but not to any other food. Allergies may get worse over time. For example, childhood rag-weed allergy may progress to year-round dust and pollen allergy. On the other hand, a person may lose allergic sensitivity. Infant or childhood atopic dermatitis disappears in almost all people. More commonly, what seems to be loss of sensitivity is instead a reduced exposure to allergens or an increased tolerance for the same level of symptoms.

While allergy to specific allergens is not inherited, the likelihood of developing some type of allergy seems to be, at least for many people. If neither parent has allergies, the chances of a child developing allergy is approximately 10-20%; if one parent has allergies, it is 30-50%; and if both have allergies, it is 40-75%. One source of this genetic predisposition is in the ability to produce higher levels of IgE in response to allergens. Those who produce more IgE will develop a stronger allergic sensitivity.

COMMON ALLERGENS. The most common air-borne allergens are the following:

• plant pollens
• animal fur and dander
• body parts from house mites (microscopic creatures found in all houses)
• house dust
• mold spores
• cigarette smoke
• solvents
• cleaners

Common food allergens include the following:

The following types of drugs commonly cause allergic reactions:

• penicillin or other antibiotics
• flu vaccines
• tetanus toxoid vaccine
• gamma globulin

Common causes of contact dermatitis include the following:

• poison ivy, oak, and sumac
• nickel or nickel alloys
• latex

Insects and other arthropods whose bites or stings typically cause allergy include the following:

• bees, wasps, and hornets
• mosquitoes
• fleas
• scabies

Symptoms depend on the specific type of allergic reaction. Allergic rhinitis is characterized by an itchy, runny nose, often with a scratchy or irritated throat due to post-nasal drip. Inflammation of the thin membrane covering the eye (allergic conjunctivitis ) causes redness, irritation, and increased tearing in the eyes. Asthma causes wheezing, coughing, and shortness of breath. Symptoms of food allergies depend on the tissues most sensitive to the allergen and whether the allergen was spread systemically by the circulatory system. Gastrointestinal symptoms may include swelling and tingling in the lips, tongue, palate or throat; nausea; cramping; diarrhea; and gas. Contact dermatitis is marked by reddened, itchy, weepy skin blisters, and an eczema that is slow to heal. It sometimes has a characteritic man-made pattern, such as a glove allergy with clear demarkation on the hands, wrist, and arms where the gloves are worn, or on the earlobes by wearing earrings.

Whole body or systemic reactions may occur from any type of allergen, but are more common following ingestion or injection of an allergen. Skin reactions include the raised, reddened, and itchy patches called hives that characteristically blanch with pressure and resolve within twenty-four hours. A deeper and more extensive skin reaction, involving more extensive fluid collection and pain, is called angioedema. This usually occurs on the extremities, fingers, toes, and parts of the head, neck, and face. Anaphylaxis is marked by airway constriction, blood pressure drop, widespread tissue swelling, heart rhythm abnormalities, and in some cases, loss of consciousness. Other syptoms may include, dizziness, weakness, seizures, coughing, flushing, or cramping. The symptoms may begin within five minutes after exposure to the allergen up to one hour or more later. Mast cells in the tissues and basophils in the blood release mediators that give rise to the clinical symptoms of this IgE-mediated hypersensitivity reaction. Commonly, this is associated with allergies to medications, foods, and insect venoms. In some individuals, anaphylaxis can occur with exercise, plasma exchange, hemodialysis, reaction to insulin, contrast media used in certain types of medical tests, and rarely during the administration of local anesthetics.

Diagnosis

Allergies can often be diagnosed by a careful medical history, matching the onset of symptoms to the exposure to possible allergens. Allergy is suspected if the symptoms presented are characteristic of an allergic reaction and this occurs repeatedly upon exposure to the suspected allergen. Allergy tests can be used to identify potential allergens, but these must be supported by eveidence of allergic responses in the patient's clinical history.

Skin tests

Skin tests are performed by administering a tiny dose of the suspected allergen by pricking, scratching, puncturing or injecting the skin. The allergen is applied to the skin as an auqeous extract, usually on the back, forearms, or top of the thighs. Once in the skin, the allergen may produce a classic immune wheal and flare response (a skin lesion with a raised, white, compressible area surrounded by a red flare). The tests usually begin with prick tests or patch tests that expose the skin to small amounts of allergen to observe the response. A positive reaction will occur on the skin even if the allergen is at levels normally encountered in food or in the airways. Reactions are usually evaluated approximately fifteen minutes after exposure. Intradermal skin tests involved injection of the allergen into the dermis of the skin. These tests are more sensitive and are used for allergies associated with risk of death, such as allergies to antibiotics.

Allergen-Specific IgE Measurement

Tests that measure allergen-specific IgE antibodies generally follow a basic method. The allergen is bound to a solid support, either in the form of a cellulose sponge, microtiter plate, or paper disk. The patient's serum is prepared from a blood sample and is incubated with the solid phase. If allergen specific IgE antibodies are present, they will bind to the solid phase and be retained there when the rest of the serum is washed away. Next, an labeled antibody against the IgE is added and will bind to any IgE on the solid phase. The excess is washed away and the levels of IgE are determined. The commonly used RAST test (radio allergo sorbent test) employed radio-labeled Anti-IgE antibodies. Updated methods now incorporate the use of enyzme-labeled antibodies in ELISA assays (enzyme-linked immunosorbent assays).

Total Serum IgE

The total level of IgE in the serum is commonly measured with a two-site immunometric assay. Some research indicates that there is a higher level of total serum IgE in allergic as compared to non-allergic people. However, this may not always be the case as there is considerable overlap between the two groups. This test is useful for the diagnosis of allergic fungal sinusitis and bronchopulmonary aspergillosis. Other conditions that are not allergic in nature may give rise to higher IgE levels such as smoking, AIDS, infection with parasites, and IgE myeloma.

Provocation tests

These tests involve the administration of allergen to elicit an immune response. Provocation tests, most commonly done with airborne allergens, present the allergen directly through the route normally involved. Delayed allergic contact dermatitis diagnosis involves similar methods by application of a skin patch with allergen to induce an allergic skin reaction. Food allergen provocation tests require abstinence from the suspect allergen for two weeks or more, followed by ingestion of a measured amount of the test substance administered as an opaque capsule along with a placebo control. Provocation tests are not used if anaphylaxis is is a concern due to the patient's medical history.

Future diagnostic methods

Attempts have been made for direct measurement of immune mediators such as histamine, eosinophil cationic protein (ECP), and mast cell tryptase. Another, somewhat controversial,test is electrodermal testing or electro-acupuncture allergy testing. This test has been used in Europe and is under investigation in the United States, though not approved by the Food and Drug Administration. An electric potential is applied to the skin, the allergen presented, and the electrical resistance observed for changes. This method has not been verified.

Treatment

Avoiding allergens is the first line of defense to reduce the possibility of an allergic attack. It is helpful to avoid environmental irritants such as tobacco smoke, perfumes, household cleaning agents, paints, glues, air fresheners, and potpourri. Nitrogen dioxide from poorly vented gas stoves, woodburning stoves, and artificial fireplaces has also been linked to poor asthma control. Dust mite control is particularly important in the bedroom areas by use of allergen-impermeable covers on mattress and pillows, frequent washing of bedding in hot water, and removal of items that collect dust such as stuffed toys. Mold growth may be reduced by lowering indoor humidity, repair of house foundations to reduce indoor leaks and seepage, and installing exhaust systems to ventilate areas where steam is generated such as the bathroom or kitchen. Allergic individuals should avoid pet allergens such as saliva, body excretions, pelts, urine, or feces. For those who insist on keeping a pet, restriction of the animal's activity to certain areas of the home may be beneficial.

Complete environmental control is often difficult to accomplish, hence therapuetic interventions may become necessary. A large number of prescription and over-the-counter drugs are available for treatment of immediate hypersensitivity reactions. Most of these work by decreasing the ability of histamine to provoke symptoms. Other drugs counteract the effects of histamine by stimulating other systems or reducing immune responses in general.

Antihistamines

Antihistamines block the histamine receptors on nasal tissue, decreasing the effect of histamine released by mast cells. They may be used after symptoms appear, though they may be even more effective when used preventively, before symptoms appear. Antihistamines help reduce sneezing, itching, and rhinorrhea. A wide variety of antihistamines are available.

Older, first generation antihistamines often produce drowsiness as a major side effect, as well as dry mouth, tachycardia, blurred vision, constipation, and lower the threshold for seizures. These medications also have similar effects to a alcohol and care should be taken when operating motor vehicles, as individuals may not be aware that they are impaired. Such anti-histamines include the following:

• diphenhydramine (Benadryl and generics)
• chlorpheniramine (Chlor-trimeton and generics)
• brompheniramine (Dimetane and generics)
• clemastine (Tavist and generics)

Newer antihistamines that do not cause drowsiness or pass the blood-brain barrier are available by prescription and include the following:

• loratidine (Claritin)
• cetirizine (Zyrtec)
• fexofenadine (Allegra)

Desloratadine (Clarinex) was approved in 2004 in syrup form for children two years and older for seasonal allergies and for hives of unknown cause in children as young as six months. It is the only nonsedating antihistamine approved as of 2004 for children as young as six months.

Hismanal has the potential to cause serious heart arrhythmias when taken with the antibiotic erythromycin, the antifungal drugs ketoconazole and itraconazole, or the antimalarial drug quinine. Taking more than the recommended dose of Hismanal can also cause arrhythimas. Seldane (terfenadine), the original non-drowsy antihistamine, was voluntarily withdrawn from the market by its manufacturers in early 1998 because of this potential and because of the availability of an equally effective, safer alternative drug, fexofenadine.

Decongestants

Decongestants constrict blood vessels to the mucosa to counteract the effects of histamine. This decreases the amount of blood in the nasopahryngeal and sinus mucosa and reduces swelling. Nasal sprays are available that can be applied directly to the nasal lining and oral systematic preparations are available. Decongestants are stimulants and may cause increased heart rate and blood pressure, headaches, insomnia, agitation, and difficulty emptying the bladder. Use of topical decongestants for longer than several days can cause loss of effectiveness and rebound congestion, in which nasal passages become more severely swollen than before treatment.

Topical corticosteroids

Topical corticosteroids reduce mucous membrane inflammation by decreasing the amount of fluid moved from the vascular spaces into the tissues. These medications reduce the recruitment of inflammatory cells as well as the synthesis of cytokines. They are available by prescription. Allergies tend to become worse as the season progresses because the immune system becomes sensitized to particular antigens and can produce a faster, stronger response. Topical corticosteroids are especially effective at reducing this seasonal sensitization because they work more slowly and last longer than most other medication types. As a result, they are best started before allergy season begins. Side effects are usually mild, but may include headaches, nosebleeds, and unpleasant taste sensations.

Bronchodilators or metered-dose inhalers (MDI)

Because allergic reactions involving the lungs cause the airways or bronchial tubes to narrow, as in asthma, bronchodilators, which cause the smooth muscle lining the airways to open or dilate, can be very effective. When inhalers are used, it is important that the patient be educated in the proper use of these medications. The inhaler should be shaken, and the patient should breathe out to expel air from the lungs. The inhaler should be placed at least two finger-breadths in front of the mouth. The medication should be aimed at the back of the throat, and the inhaler activated while breathing in quite slowly 3-4 seconds. The breath should be held for at least ten seconds, and then expelled. At least thirty to sixty seconds should pass before the inhaler is used again. Care should be taken to properly wash out the mouth and brush the teeth following use, as residual medication remains in this area with only a small amount actually reaching the lungs. Some bronchodilators used to treat acute asthma attacks include adrenaline, albuterol, Maxair, Proventil, or other "adrenoceptor stimulants," most often administered as aerosols. Successfully managing asthma and allergies can reduce the use of inhalers. This is done through good communication between the physician and patient, self-management with written action plans, avoiding allergy triggers, and through the use of preventive medications such as montelukast.

Anticholinergics

Ipratropium bromide (atrovent) and atropine sulfate are achticholinergic drugs used for the treatment of asthma. Ipratropium is used for treating asthmatics in emergency situations with a nebulizer.

Nonsteroidal drugs

MAST CELL STABILIZERS. Cromolyn sodium prevents the release of mast cell granules, thereby preventing the release of histamine and other chemicals contained in them. It acts as a preventive treatment if it is begun several weeks before the onset of the allergy season. It can also be used for year round allergy prevention. Cromolyn sodium is available as a nasal spray for allergic rhinitis and in aerosol (a suspension of particles in gas) form for asthma.

LEUKOTRIENE MODIFIERS. These medications are useful for individuals with aspirin sensitivity, sinusitis, polposis, urticaria. Examples include zafirlukast (Accolate), montelukast (Singulair), and zileuton (Zyflo). When zileuton is used, care must be taken to measure liver enzymes.

Immunotherapy

In this form of therapy, allergen is injected into the skin in increasing doses over a specific period of time. This may be helpful for patients who do not respond to medications or avoidance of allergens in the environment. This type of therapy may reduce the need for medications. A 2004 study recommended that children who have severe reactions to insect sting receive immunotherapy to protect them against future stings.

Treatment of contact dermatitis

An individual suffering from contact dermatitis should initially take steps to avoid possible sources of exposure to the offending agent. Calamine lotion applied to affected skin can reduce irritation somewhat, as can cold water compresses. Side effects of topical agents may include over-drying of the skin. In the case of acute contact dermatitis, short-term oral corticosteroid therapy may be appropriate. Moderately strong coricosteroids can also be applied as a wrap for twenty-four hours. Health care workers are especially at risk for hand eruptions due to glove use.

Treatment of anaphylaxis

The emergency condition of anaphylaxis is treated with injection of adrenaline, also known as epinephrine. People who are prone to anaphylaxis because of food or insect allergies often carry an "Epi-pen" containing adrenaline in a hypodermic needle. Other medications may be given to aid the action of the epipen. Prompt injection can prevent a more serious reaction from developing. Particular care should be taken to assess the affected individual's airway status, and he or she should be placed in a recumbent pose and vital signs determined. If a reaction resulted from insect sting or an injection, a tourniquet may need to be placed proximal to the area where the agent penetrated the skin. This should then be released at intervals of ten minutes at a time, for one to two minutes duration. If the individual does not respond to such interventions, then emergency treatment is appropriate.

Alternative treatment

Any alternative treatment for allergies begins with finding the cause and then helping the patient to avoid or eliminate the allergen, although this is not always possible. As with any alternative therapy, a physician should be consulted before initiating a new form of treatment. Education on the use of alternative agents is critical, as they are still "drugs" even though they are derived from natural sources. Various categories of alternative remedies may be helpful in allergy treatment, including:

• antihistamines: vitamin C and the bioflavonoid hesperidin act as natural anithistamines.
• decongestants: vitamin C, the homeopathic reme-dies Ferrum phosphoricum and Kali muriaticum (used alternately), and the dietary supplement N-acetylcysteine are believed to have decongestant effects.
• mast cell stabilizers: the bioflavonoids quercetin and hesperidin may help stabilize mast cells.
• immunotherapy: the herbs echinacea (Echinacea spp.) and astragalus or milk-vetch root (Astragalus membranaceus ) may possibly help to strengthen the immune system.
• bronchodilators: the herbal remedies ephedra (Ephedra sinica, also known as ma huang in traditional Chinese medicine), khellin (Ammi visnaga ) and cramp bark (Viburnum opulus ) are believed to help open the airways.

Treatment of contact dermatitis

A variety of herbal remedies, either applied topically or taken internally, may possibly assist in the treatment of contact dermatitis. A poultice (crushed herbs applied directly to the affected area) made of jewelweed (Impatiens spp.) or chickweed (Stellaria media ) may soothe the skin. A cream or wash containing calendula (Calendula officinalis ), a natural antiseptic and anti-inflammatory agent, may help heal the rash when applied topically. Homeopathic treatment may include such remedies as Rhus toxicodendron, Apis mellifica, or Anacardium taken internally. A qualified homeopathic practitioner should be consulted to match the symptoms with the correct remedy. Care should be taken with any agent taken internally.

Prognosis

Allergies can improve over time, although they often worsen. While anaphylaxis and severe asthma are life-threatening, other allergic reactions are not. Learning to recognize and avoid allergy-provoking situations allows most people with allergies to lead normal lives.

Prevention

Avoiding allergens is the best means of limiting allergic reactions. For food allergies, there is no effective treatment except avoidance. By determining the allergens that are causing reactions, most people can learn to avoid allergic reactions from food, drugs, and contact allergens such as poison ivy or latex. The government will help now, since passing the Food Allergen Labeling and Consumer Protection Act in 2004. Beginning January 1, 2006, food manufacturers will be required to clearly state if a product contains any of the eight major food allergens that are responsible for more than 90% of allergic reactions to foods. These are milk, eggs, peanuts, tree nuts, fish, shellfish, wheat, and soy.

Airborne allergens are more difficult to avoid, although keeping dust and animal dander from collecting in the house may limit exposure. Cromolyn sodium can prevent mast cell degranulation, thereby limiting the allergic response.

Immunotherapy, also known as desensitization or allergy shots, alters the balance of antibody types in the body, thereby reducing the ability of IgE to cause allergic reactions. Immunotherapy is preceded by allergy testing to determine the precise allergens responsible. Injections involve very small but gradually increasing amounts of allergen, over several weeks or months, with periodic boosters. Full benefits may take up to several years to achieve and are not seen at all in about one in five patients. Individuals receiving all shots will be monitored closely following each shot because of the small risk of anaphylaxis, a condition that can result in difficulty breathing and a sharp drop in blood pressure.

Other drugs, such as leukotriene modifiers, are used to prevent asthma attacks and in the long-term management of allergies and asthma.

Resources

BOOKS

Hans-Uwe, Simon, editor. CRC Desk Reference for Allergy and Asthma. Boca Raton: CRC Press, 2000.

Kemp, Stephen F., and Richard Lockey, editors. Diagnostic Testing of Allergic Disease. New York: Marcel Dekker, Inc., 2000.

Lieberman, Phil, and Johh Anderson, editors. Allergic Diseases: Diagnosis and Treatment. 2nd ed. Totowa: Humana Press, Inc., 2000.

PERIODICALS

"Children With Serious Insect-sting Allergies Should Get Shots." Drug Week (September 3, 2004): 19.

"FDA Approves Clarinex Syrup for Allergies and Hives in Children." Biotech Week (September 29, 2004): 617.

"President Bush Signs Bill that Will Benefit Millions With Food Allergies." Immunotherapy Weekly (September 1, 2004): 50.

"What's New in: Asthma and Allergic Rhinitis." Pulse (September 20, 2004): 50.

Allergies

Definition

Allergies are abnormal reactions of the immune system that occur in response to otherwise harmless substances.

Description

Allergies are among the most common of medical disorders. It is estimated that 60 million Americans, or more than one in every five people, suffer from some form of allergy, with similar proportions throughout much of the rest of the world. Allergy is the single largest reason for school absence and is a major source of lost productivity in the workplace.

Allergies are a type of immune reaction. Normally, the immune system responds to foreign microorganisms, or particles, like pollen or dust, by producing specific proteins, called antibodies, that are capable of binding to identifying molecules, or antigens, on the foreign particle. This reaction between antibody and antigen sets off a series of reactions designed to protect the body from infection. When this same series of reactions is triggered by harmless, everyday substances, it is called an allergy. The substance that causes the allergy is called an allergen.

All allergic reactions involve a special set of cells in the immune system known as mast cells. Mast cells, found in the lining of the nasal passages and eyelids, display a special type of antibody, called immunoglobulin type E (IgE), on their surface. Inside, mast cells store reactive chemicals in small packets, called granules. When the antibodies encounter allergens, they trigger release of the granules, which spill out their chemicals onto neighboring cells, including blood vessels and nerve cells. One of these chemicals, histamine, binds to the surfaces of these other cells, through special proteins called histamine receptors. Interaction of histamine with receptors on blood vessels causes neighboring cells to become leaky, leading to the fluid collection in the body's tissues, swelling, and increased redness characteristic of a runny nose and red, irritated eyes. Histamine also stimulates pain receptors, causing the itchy nose, eyes, and throat common in allergic rhinitis . In the gastrointestinal tract, these reactions lead to swelling and irritation of the intestinal lining, which causes the cramping and diarrhea typical of food allergy. Allergens that enter the circulation may cause hives , angioedema, anaphylaxis , or atopic dermatitis . Allergens on the skin usually cause a delayed hypersensitivity reaction. This type of allergic response may develop over several days following contact with the allergen, and symptoms may persist for a week or more.

Demographics

According to the National Institute of Allergy and Infectious Diseases, annually, more than 50 million Americans suffer from allergic diseases, with approximately 36 million suffering from allergic rhinitis . Upto 6 percent of the general population suffers from an allergy to latex, and children with spina bifida who have had multiple surgical procedures are at higher risk for allergic reactions to latex. Atopic dermatitis is one of the most common skin conditions and occurs commonly in infants and children. Prevalence in the United States is about 10 percent. Food allergies occur in 8 percent of children aged six years and younger. Peanut or other nut allergies affect about 3 million Americans and produce the most severe reactions. Acute allergic hives affect from 10 percent to 20 percent of Americans at some time during their lifetime, and half of those affected have symptoms for more than six months. Allergies to stinging insects occur in about 3.5 percent of Americans. According to the American Academy of Allergy, Asthma , and Immunology, if one parent has an allergic disease, a child has a 48 percent risk of developing allergies. If both parents have allergies, risk increases to 70 percent.

Causes and symptoms

Allergens enter the body through four main routes: the airways, the skin, the gastrointestinal tract, and the circulatory system.

Airborne allergens cause the sneezing, runny nose, and itchy, bloodshot eyes of allergic rhinitis (hay fever). Airborne allergens can also affect the lining of the lungs, causing asthma, or the conjunctiva of the eyes, causing allergic conjunctivitis . The most common airborne allergens are the following:

• plant pollens
• animal fur and dander
• body parts and excrement from dust mites (microscopic creatures found in all houses)
• excrement from cockroaches
• house dust
• mold spores
• cigarette smoke
• solvents
• cleaners

Allergens in food can cause itching and swelling of the lips and throat, cramps, and diarrhea. When absorbed into the bloodstream, they may cause hives (urticaria) or more severe reactions involving recurrent, non-inflammatory swelling of the skin, mucous membranes, organs, and brain (angioedema). Some food allergens may cause anaphylaxis, a potentially life-threatening condition marked by tissue swelling, airway constriction, and drop in blood pressure. Common food allergens include the following:

• nuts, especially peanuts, walnuts, and brazil nuts
• fish, mollusks, and shellfish
• eggs
• wheat
• milk
• food additives and preservatives

In contact with the skin, allergens can cause reddening, itching, and blistering, called contact dermatitis . Skin reactions can also occur from allergens introduced through the airways or gastrointestinal tract. This type of reaction is known as atopic dermatitis. Dermatitis may arise from an allergic response (such as from poison ivy ) or exposure to an irritant causing nonimmune damage to skin cells (such as soap, cold, and chemical agents). Injection of allergens, from insect bites and stings or drug administration, can introduce allergens directly into the circulation, where they may cause system-wide responses (including anaphylaxis), as well as the local ones of swelling and irritation at the injection site.

Common causes of contact dermatitis include the following:

• poison ivy, oak, and sumac
• nickel or nickel alloys
• latex

Insects and other arthropods whose bites or stings typically cause allergy include the following:

• bees, wasps, and hornets
• mosquitoes
• fleas

The following types of drugs commonly cause allergic reactions:

• penicillin or other antibiotics
• flu vaccines
• tetanus toxoid vaccine
• gamma globulin

Children and adolescents with allergies are not equally sensitive to all allergens. Some may have severe allergic rhinitis but no food allergies, for instance, or be extremely sensitive to nuts but not to any other food. Allergies may get worse over time. For example, childhood ragweed allergy may progress to year-round dust and pollen allergy. On the other hand, a child may outgrow allergic sensitivity. Infant or childhood atopic dermatitis disappears in almost all people. More commonly, what seems to be loss of sensitivity is instead a reduced exposure to allergens or an increased tolerance for the same level of symptoms.

Symptoms depend on the specific type of allergic reaction. Allergic rhinitis is characterized by an itchy, runny nose, sneezing, and often a scratchy or irritated throat due to postnasal drip. Inflammation of the thin membrane covering the eye (allergic conjunctivitis) causes redness, irritation, and increased tearing in the eyes. Asthma causes wheezing, coughing, and shortness of breath. Symptoms of food allergies depend on the tissues most sensitive to the allergen and whether the allergen spread systemically by the circulatory system. Gastrointestinal symptoms may include swelling and tingling in the lips, tongue, palate or throat; nausea ; cramping; diarrhea; and gas. Contact dermatitis is marked by red, itchy, weepy skin blisters, and an eczema that is slow to heal. It sometimes has a characteristic pattern from the object containing the allergen, such as a glove allergy with clear demarcation on the hands, wrist, and arms where the gloves are worn, or on the earlobes by wearing earrings.

Whole-body or systemic reactions may occur from any type of allergen but are more common following ingestion or injection of an allergen. Skin reactions include the raised, red, and itchy patches called hives that characteristically blanch with pressure and resolve within 24 hours. A deeper and more extensive skin reaction, involving more extensive fluid collection and pain, is called angioedema. This response usually occurs on the extremities, fingers, toes, and parts of the head, neck, and face. Anaphylaxis is marked by airway constriction, blood pressure drop, widespread tissue swelling, heart rhythm abnormalities, and in some cases, loss of consciousness. Other symptoms may include dizziness , weakness, seizures, coughing, flushing, or cramping. The symptoms may begin within five minutes after exposure to the allergen up to one hour or more later. Commonly, this is associated with allergies to medications, foods, and insect venoms. In some individuals, anaphylaxis can occur with exercise , plasma exchange, hemodialysis, reaction to insulin, radiocontrast media used in certain types of medical tests, and on rare occasions during the administration of local anesthetics.

When to call the doctor

Parents should consult a physician when a child has repeated and prolonged symptoms. Allergic rhinitis may be mistaken for a cold or other upper respiratory infection. Usually, a fever indicates an infection. Food allergies and allergies to insect stings or medications can be especially dangerous, causing anaphylactic reactions that require emergency treatment.

Diagnosis

Allergies can often be diagnosed by a careful medical history, matching the onset of symptoms to the exposure to possible allergens. Allergy is suspected if the symptoms presented are characteristic of an allergic reaction, and this occurs repeatedly upon exposure to the suspected allergen. Allergy tests can also be conducted to determine allergens.

Skin tests

Skin tests are performed by administering a tiny dose of the suspected allergen by pricking, scratching, puncturing, or injecting the skin. The allergen is applied to the skin as an aqueous extract, usually on the back, forearms, or top of the thighs. Once in the skin, the allergen may produce a classic immune wheal and flare response (a skin lesion with a raised, white, compressible area surrounded by a red flare). The tests usually begin with prick tests or patch tests that expose the skin to small amounts of allergen to observe the response. A positive reaction occurs on the skin even if the allergen is at levels normally encountered in food or in the airways. Reactions are usually evaluated approximately 15 minutes after exposure. Intradermal skin tests involve injection of the allergen into the dermis of the skin. These tests are more sensitive and are used for allergies associated with risk of death, such as allergies to antibiotics. Skin testing may be painful for children.

Provocation tests

These tests involve the administration of allergen to elicit an immune response. Provocation tests, most commonly done with airborne allergens, present the allergen directly through the route normally involved. Delayed

allergic contact dermatitis diagnosis involves similar methods by application of a skin patch with allergen to induce an allergic skin reaction. Food allergen provocation tests require abstinence from the suspect allergen for two weeks or more, followed by ingestion of a measured amount of the test substance administered as an opaque capsule along with a placebo control. Provocation tests are not used if anaphylaxis is a concern given the patient's medical history.

Treatment

Avoiding allergens is the first line of defense to reduce the possibility of an allergic attack. However, complete environmental control is often difficult to accomplish; hence, therapeutic interventions are usually necessary. A large number of prescription and over-the-counter drugs are available for treatment of immediate hypersensitivity reactions. Most of these products work by decreasing the ability of histamine to provoke symptoms. Other drugs counteract the effects of histamine by stimulating other systems or reducing immune responses in general.

Antihistamines

Antihistamines are drugs used to treat the symptoms of allergic rhinitis by blocking the action of histamine, a chemical released by the immune system in allergic reactions. Antihistamines are available as prescription and over-the-counter tablets, topical gels or creams, nasal sprays, and eye drops.

Commonly used antihistamines include the following:

• diphenhydramine (Benadryl)
• loratadine (Claritin)
• cetirizine (Zyrtec)
• fexofenadine (Allegra)
• clemastine fumarate (Tavist)
• chlorpheniramine (Chlor Trimeton)
• brompheniramine (Dimetapp)

Decongestants

Decongestants dry up nasal passage tissues and reduce swollen nasal membranes so as to relieve congestion. Decongestants are available as nasal sprays or drops, oral tablets, or syrups. Decongestants are stimulants and may cause increased heart rate and blood pressure, headaches, and agitation. Use of nasal spray decongestants for longer than four to five days can cause loss of effectiveness and rebound congestion, in which nasal passages become more severely swollen than before treatment. Saline nasal sprays, which do not contain decongestants, may be used for longer periods of time to help congestion and nasal passage irritation.

Commonly used decongestants include the following:

• oxymetazoline (Afrin)
• pseudoephedrine (Sudafed)
• phenylephrine (Neo Synephrine)

Corticosteroids

Corticosteroids reduce mucous membrane inflammation and are available by prescription and taken as a series of oral tablets. Corticosteroids are also available as nasal sprays. Allergies tend to become worse as the season progresses because the immune system becomes sensitized to particular antigens and can produce a faster, stronger response. Corticosteroids are especially effective at reducing this seasonal sensitization because they work more slowly and last longer than most other medication types. Side effects may include headaches, nosebleeds, and unpleasant taste sensations. Long-term use of oral corticosteroids may cause more serious side effects, such as weight gain, cataracts, weakening bones, high blood pressure, elevated blood sugar, and easy bruising.

Mast cell stabilizers

Cromolyn sodium prevents the release of mast cell granules, thereby preventing release of histamine and the other chemicals contained in them. Cromolyn sodium is available in nasal sprays or via an inhaler. It is most frequently prescribed when allergic rhinitis is accompanied by asthma.

Immunotherapy

Immunotherapy, also known as desensitization therapy or allergy shots , alters the balance of antibody types in the body, thereby reducing the ability of IgE to cause allergic reactions. Immunotherapy is preceded by allergy testing to determine the precise allergens responsible. Injections involve very small but gradually increasing amounts of allergen, over several weeks or months, with periodic boosters. Full benefits may take up to several years to achieve and are not seen at all in about one in five patients. Individuals receiving all shots are monitored closely following each shot because of the small risk of anaphylaxis, a condition that can result in difficulty breathing and a sharp drop in blood pressure.

Treatment of contact dermatitis

An individual suffering from contact dermatitis should initially take steps to avoid possible sources of exposure to the offending agent. Calamine lotion applied to affected skin can reduce irritation somewhat, as can cold-water compresses. Topical antihistamine and corticosteroid sprays, gels, and creams are available to reduce itching. Side effects of topical agents may include overdrying of the skin. In the case of acute contact dermatitis, short-term oral corticosteroid therapy may be appropriate. Moderately strong corticosteroids can also be applied as a wrap for 24 hours. Healthcare workers are especially at risk for hand eruptions due to latex glove use.

Treatment of anaphylaxis

The emergency condition of anaphylaxis is treated with injection of adrenaline, also known as epinephrine. Children and adolescents who are prone to anaphylaxis because of food or insect allergies often carry an Epipen containing adrenaline in a hypodermic needle. Other medications may be given to aid the action of the Epipen. Prompt injection can prevent a more serious reaction from developing. Particular care should be taken to assess the affected child's airway status, and he or she should be placed in a recumbent pose and vital signs determined. Emergency treatment may be required for severe reactions.

Nutritional concerns

For children and adolescents with food allergies, all foods must be monitored to make sure that the allergen is not an ingredient or was not used during preparation. In individuals with severe food allergies to peanuts, peanut oil used to fry foods, or even the fumes produced during cooking with peanut oil have been known to cause anaphylactic shock.

Parents whose children have allergies to foods, like milk and gluten, which are common ingredients in many other foods, can purchase gluten-free foods and lactose-free foods in most grocery stores. Cookbooks dealing with allergies to these foods are also available.

Prognosis

Allergies can improve over time, although they often worsen. While anaphylaxis and severe asthma are life threatening, other allergic reactions are not. Learning to recognize and avoid allergy-provoking situations allows most children and adolescents with allergies to lead normal lives.

Prevention

Avoiding allergens is the best means of limiting allergic reactions. For food allergies, there is no effective treatment except avoidance. By determining the allergens that are causing reactions most people can learn to avoid allergic reactions from food, drugs, and contact allergens such as poison ivy or latex. Airborne allergens are more difficult to avoid. Preventive measures for airborne allergens include the following:

• staying indoors with windows closed during the morning hours, when pollen levels are highest
• keeping car windows up while driving
• using a surgical face mask when outside
• avoiding uncut fields
• learning which trees are producing pollen in which seasons and avoiding forests at the height of pollen season
• washing clothes and hair after being outside
• regularly cleaning air conditioner filters in the home
• using electrostatic filters for central air conditioning

For mold spores, the following steps will help:

• keeping the house dry through ventilation and use of dehumidifiers
• using a disinfectant such as diluted bleach to clean surfaces such as bathroom floors and walls
• having air ducts cleaned and disinfected
• cleaning and disinfecting air conditioners and coolers
• throwing out moldy or mildewed books, shoes, pillows, or furniture

For house dust, the following steps will help:

• vacuuming frequently and changing the bag regularly; using a bag with small pores to catch extra-fine particles
• cleaning floors and walls with a damp mop
• installing electrostatic filters in heating and cooling ducts and changing all filters regularly

For animal dander, the following steps will help:

• avoiding contact if possible
• washing hands after contact
• vacuuming frequently
• keeping pets out of the bedroom and off furniture, rugs, and other dander-catching surfaces
• bathing and grooming pets frequently

Parents may find it helpful to keep an allergy journal for their child to track occurrence of allergic responses. For seasonal allergic rhinitis, they may use a calendar to note when symptoms begin and end. Documenting the level of seasonal allergens at the time can help determine when seasonal allergies tend to occur and what allergens affect the child. Local weather reports on television and on Web sites provide detailed allergen maps of pollen and mold/mildew spores. Antihistamines can then be taken as a preventive measure before symptoms begin each season. For children with allergies to foods, keeping a journal of foods eaten can help identify specific food allergens.

Parental concerns

For children who resist taking pills, many antihistamines are available as flavored chewable tablets, tablets that easily dissolve on the tongue, or flavored syrups. Because many over-the-counter allergy medicines contain multiple drugs, parents should be sure to read the prescribing and dosage information for any antihistamine their children are taking to ensure safe use.

Parents of children and adolescents with severe food and insect sting allergies that might result in sudden anaphylactic reactions should make sure that their children and any other family members and caregivers fully understand the severity of the allergic response and the need for immediate administration of epinephrine. Parents should consider having children with these severe allergies wear a medical alert bracelet.

Children with severe food allergies to whole food groups, such as milk or wheat, may require dietary management by a dietitian or nutritionist to ensure they receive the proper nutrients and a well-balanced diet. Breastfeeding mothers of highly allergic infants may need to eliminate suspected food allergens from their diets, because food proteins ingested by a mother can be transferred to the infant via breast milk. Special formulas are available for infants sensitive to breast milk, cow's milk, and soy milk.

KEY TERMS

Allergen —A foreign substance that provokes an immune reaction or allergic response in some sensitive people but not in most others.

Allergic rhinitis —Swelling and inflammation of the nasal membranes caused by sensitivity to airborne matter like pollen or cat hair.

Anaphylaxis —Also called anaphylactic shock; a severe allergic reaction characterized by airway constriction, tissue swelling, and lowered blood pressure.

Angioedema —Patches of circumscribed swelling involving the skin and its subcutaneous layers, the mucous membranes, and sometimes the organs frequently caused by an allergic reaction to drugs or food. Also called angioneurotic edema, giant urticaria, Quincke's disease, or Quincke's edema.

Antibody —A special protein made by the body's immune system as a defense against foreign material (bacteria, viruses, etc.) that enters the body. It is uniquely designed to attack and neutralize the specific antigen that triggered the immune response.

Antigen —A substance (usually a protein) identified as foreign by the body's immune system, triggering the release of antibodies as part of the body's immune response.

Asthma —A disease in which the air passages of the lungs become inflamed and narrowed, causing wheezing, coughing, and shortness of breath.

Atopic dermatitis —An intensely itchy inflammation often found on the face, in the bend of the elbow, and behind the knees of people prone to allergies. In infants and young children, this condition is called infantile eczema.

Conjunctivitis —Inflammation of the conjunctiva, the mucous membrane covering the white part of the eye (sclera) and lining the inside of the eyelids also called pinkeye.

Contact dermatitis —Skin inflammation as a result of contact with a foreign substance.

Granules —Small packets of reactive chemicals stored within cells.

Histamine —A substance released by immune system cells in response to the presence of an allergen. It stimulates widening of blood vessels and increased porousness of blood vessel walls so that fluid and protein leak out from the blood into the surrounding tissue, causing localised inflammation of the tissue.

Immune hypersensitivity reaction —An allergic reaction that is mediated by mast cells and occurs within minutes of allergen contact.

Mast cells —A type of immune system cell that is found in the lining of the nasal passages and eyelids. It displays a type of antibody called immunoglobulin type E (IgE) on its cell surface and participates in the allergic response by releasing histamine from intracellular granules.

Resources

BOOKS

Borrel, Marie. 60 Tips: Allergies. London: Hachette Illustrated, 2004.

Cross, Linda. How to Manage Your Child's Life-Threatening Food Allergies. Lake Forest, CA: Plumtree Press, 2004.

Dadamo, Peter J. Eat Right for Your Type Allergies. East Rutherford, NJ: Penguin Group, 2005.

Ford, Jean. Breathe Easy: A Teen's Guide to Allergies and Asthma. Broomall, PA: Mason Crest Publishers, 2005.

Taylor, R., et al. Allergy Relief and Prevention, 3rd. ed. Vancouver: Hartley and Marks, 2000.

PERIODICALS

Ansel, K. "Working with the Food-Allergic Child. Today's Dietitian. 4 (November 2002): 36–39.

ORGANIZATIONS

Allergy and Asthma Network. 3554 Chain Bridge Road, Suite 200. Web site: <www.aanma.org>.

American Academy of Allergy, Asthma, and Immunology. 611 East Wells St, Milwaukee, WI 53202. Web site: <www.aaaai.org>.

Asthma and Allergy Foundation of America. 1125 15th Street NW, Suite 502, Washington, DC 20005. Web site: <www.aafa.org/>.

WEB SITES

"All About Allergies." Nemours Foundation. Available online at <www.kidshealth.org/parent/medical/allergies/allergy.html> (accessed October 26, 2004).

Jennifer E. Sisk, MA

Allergy

Types of allergy

Role of immune system

Diagnosis and treatment

Resources

An allergy is an excessive or hypersensitive response of the immune system. The allergic reaction becomes manifest as a pathological immune reaction induced either by antibodies (immediate hypersensitivity) or by lymphoid cells (delayed type allergy). Instead of fighting off a disease-causing foreign substance, the immune system launches a complex series of actions against an irritating substance, referred to as an allergen. Allergens enter the body through four main routes: the airways, the skin, the gastrointestinal tract, and the circulatory system. Some problems include:

• Airborne allergens cause the sneezing, runny nose, and itchy, bloodshot eyes of hay fever (allergic rhinitis). Some of the most common airborne allergens are plant pollens, animal fur and dander, body parts from house mites (microscopic creatures found in all houses), house dust, mold spores, cigarette smoke, solvents, and cleaners. Airborne allergens can also affect the lining of the lungs, causing asthma, or the conjunctiva of the eyes, causing conjunctivitis (pink eye).
• Allergens in food can cause itching and swelling of the lips and throat, cramps, and diarrhea. When absorbed into the bloodstream, they may cause hives (urticaria) or more severe reactions involving recurrent, noninflammatory swelling of the skin, mucous membranes, organs, and brain (angioedema). Some food allergens may cause anaphylaxis, a potentially life-threatening condition marked by tissue swelling, airway constriction, and drop in blood pressure. Common food allergens include nuts, especially peanuts, walnuts, and Brazil nuts; fish, mollusks, and shellfish; eggs; wheat; milk; food additives and preservatives.
• In contact with the skin, allergens can cause reddening, itching, and blistering, called contact dermatitis. Skin reactions can also occur from allergens introduced through the airways or gastrointestinal tract. This type of reaction is known as atopic dermatitis.
• Injection of allergens, from insect bites and stings or drug administration, can introduce allergens directly into the circulation, where they may cause system-wide responses (including anaphylaxis), as well as the local ones of swelling and irritation at the injection site.

The symptoms of an immediate hypersensitivity begin shortly after contact and decay rapidly, while the delayed type symptoms do not reach their maximum for 24 to 48 hours and decline slowly over a period of days or weeks. An allergic reaction may be accompanied by a number of stressful symptoms, ranging from mild to severe to life threatening.

In rare cases, an allergic reaction can lead to anaphylactic shock—a condition characterized by a sudden drop in bloodpressure, difficulty in breathing, skin irritation, collapse, and possible death.

Allergies are among the most common of medical disorders. According to the American Academy of Allergy, Asthma and Immunology, as of 2005, over 50 million Americans, or more than one in every five people, suffer from some form of allergy, with similar proportions throughout much of the rest of the world. Allergies, as a group, are the single largest reason for school absence and is a major source of lost productivity in the workplace.

The immune system may produce several chemical agents that cause allergic reactions. The main group of immune system substances responsible for the symptoms of allergy includes the histamines, which are produced after an exposure to an allergen. Along with other treatments and medicines, the use of antihistamines helps to relieve some of the symptoms of allergy by blocking out histamine receptor sites. The study of allergy medicine includes the identification of the different types of allergy, immunology, and the diagnosis and treatment of allergy.

Types of allergy

The most common cause of allergy is pollens that are responsible for seasonal or allergic rhinitis. The popular name for rhinitis, hay fever, a term used since the 1830s, is inaccurate because the condition is not characterized by fever. Throughout the world during every season, pollens from grasses, trees, and weeds affect certain individuals, producing allergic reactions like sneezing, runny nose, swollen nasal tissues, headaches, blocked sinuses, and watery, irritated eyes. According to the American Academy of Allergy, Asthma, and Immunology (AAAAI), of the 40 to 50 million allergy sufferers in the United States (as of 2006), about two-thirds of them have rhinitis. Allergic diseases affect over 20% of the population in the United States, making it the sixth leading cause of chronic disease in the country.

Dust and the house dust mite constitute another major cause of allergies. While the mite itself is too large to be inhaled, its feces are about the size of pollen grains and can lead to allergic rhinitis. Other types of allergy can be traced to the fur of animals and pets, food, drugs, insect bites, and skin contact with chemical substances or odors. In the United States, there are over 12 million people who are allergic to a variety of chemicals. In some cases an allergic reaction to an insect sting or a drug reaction can cause sudden death. Serious asthma attacks are associated with seasonal rhinitis and other allergies. According to the AAAAI, about 20 million people in the United States suffer from asthma in any given year.

Role of immune system

Some people are allergic to a wide range of allergens, while others are allergic to a few or only one— while some people have no allergies. The reasons for these differences can be found in the makeup of an individual’s immune system. The immune system is the body’s defense against substances (antigens) that it recognizes as foreign to the body. Lymphocytes, a type of white blood cell, produce antibodies, which play an important role in neutralizing antigens, including bacteria, viruses, and toxins. Those antigens specifically producing an allergic response are known as allergens When an allergen first enters the body, the lymphocytes produce an antibody called immunoglobulin E (IgE). The IgE antibodies attach to mast cells, large cells that are found in connective tissue and contain histamines along with a number of other chemical substances.

Studies show that allergy sufferers produce an excessive amount of IgE, indicating a hereditary factor for their allergic responses. How individuals adjust over time to allergens in their environments also determines their degree of susceptibility to allergic disorders.

The second time any given allergen enters the body, it becomes attached to the newly-formed Y-shaped IgE antibodies. These antibodies, in turn, stimulate the mast cells to discharge its histamines and other anti-allergen substances. There are two types of histamine: H₁ and H₂. H₁ histamines travel to receptor sites located in the nasal passages, respiratory system, and skin, dilating smaller blood vessels and constricting airways. The H₂ histamines, which constrict the larger blood vessels, travel to the receptor sites found in the salivary and tear glands and in the stomach’s mucosal lining. H₂ histamines play a role in stimulating the release of stomach acid, thus creating a seasonal stomach ulcer condition.

Diagnosis and treatment

The patient’s medical history provides the primary basis for the diagnosis of allergies. Skin patch tests are sometimes used to determine exactly which potential allergens can produce a reaction in the patient. A group of substances are placed in patches under the skin; any actual allergen will raise a weal (a red, circular swelling) at the site of the patch. The weal is a circular area of swelling that itches and is reddened. The tests at times produce false positives, so that they cannot be relied on exclusively.

The simplest form of treatment is the avoidance of the allergic substance, but that is not always possible. In such cases, desensitization to the allergen is sometimes attempted by exposing the patient to slight amounts of the allergen at regular intervals.

Antihistamines, which are now prescribed and sold over the counter as a rhinitis remedy, were discovered in the 1940s. There are a number of different ones, and they either inhibit the production of hista-mine or block them at receptor sites. After the administration of antihistamines, IgE receptor sites on the mast cells are blocked, thereby preventing the release of the histamines that cause the allergic reactions. The allergens are still there, but the body’s protective actions are suspended for the period of time that the antihistamines are active. Antihistamines also constrict the smaller blood vessels and capillaries, thereby removing excess fluids. Recent research has identified specific receptor sites on the mast cells for the IgE. This knowledge makes it possible to develop medicines that will be more effective in reducing the symptoms of various allergies.

Older antihistamines often produce drowsiness as a major side effect. Such antihistamines include the following:

• Diphenhydramine (Benadryl^® and generics),
• Chlorpheniramine (Chlor-trimeton^® and generics),
• Brompheniramine (Dimetane^® and generics), and
• Clemastine (Tavist^® and generics).

Newer antihistamines that do not cause drowsiness and include the following:

• Astemizole (Hismanal^®),
• Loratidine (Claritin^®),
• Fexofenadine (Allegra^®), and
• Azelastin HCl (Astelin^®).

Hismanal^® has the potential to cause serious heart arrhythmias when taken with the antibiotic erythromycin, the antifungal drugs ketoconazole and itraconazole, or the antimalarial drug quinine. Taking more than the recommended dose of Hismanal^® can also cause arrhythimas. Seldane^® (terfenadine), the original non-drowsy antihistamine, was voluntarily withdrawn from the market by its manufacturers in early 1998 because of this potential and because of the availability of an equally effective, safer alternative drug, fexofenadine.

Corticosteroids are sometimes prescribed to allergy sufferers as anti-inflammatories. Decongestants can also bring relief, but these can be used for a short time only, since their continued use can set up a rebound effect and intensify the allergic reaction.

Decongestants constrict blood vessels to counteract the effects of histamine. Nasal sprays are available that can be applied directly to the nasal lining and oral systemic preparations are available. Decongestants are stimulants and may cause increased heart rate and blood pressure, headaches, and agitation. Use of topical decongestants for longer than several days can cause loss of effectiveness and rebound congestion, in which nasal passages become more severely swollen than before treatment.

Topical corticosteroids reduce mucous membrane inflammation and are available by prescription. Allergies tend to become worse as the season progresses because the immune system becomes sensitized to particular antigens and, thus, it can produce a faster, stronger response. Topical corticosteroids are especially effective at reducing this seasonal sensitization because they work more slowly and last longer than most other medication types. As a result, they are best started before allergy season begins. Side effects are usually mild, but may include headaches, nosebleeds, and unpleasant taste sensations.

Cromolyn sodium prevents the release of mast cell granules, thereby preventing the release of histamine and other chemicals contained in them. It acts as a preventive treatment if it is begun several weeks before the onset of the allergy season. It can also be used for year round allergy prevention. Cromolyn sodium is available as a nasal spray for allergic rhinitis and in aerosol (a suspension of particles in gas) form for asthma.

Immunotherapy, also known as desensitization or allergy shots, alters the balance of antibody types in the body, thereby reducing the ability of IgE to cause allergic reactions. Immunotherapy is preceded by allergy testing to determine the precise allergens responsible. (These tests are described in full in the article on allergy testing.) Injections involve very small but gradually increasing amounts of allergen, over several weeks or months, with periodic boosters. Full benefits may take up to several years to achieve and are not seen at all in about one in five patients. Individuals receiving all shots will be monitored closely following each shot because of the small risk of anaphylaxis, a condition that can result in difficulty breathing and a sharp drop in blood pressure.

Because allergic reactions involving the lungs cause the airways or bronchial tubes to narrow, as in asthma, bronchodilators, which cause the smooth

KEY TERMS

Allergen— An otherwise harmless substance that can cause a hypersensitive allergic response.

Anaphylactic shock— A violent, sometimes fatal, response to an allergen after initial contact.

Corticosteroids— Drugs that stimulate the adrenal gland and are highly effective in treating asthma and allergies but also have many side effects.

Decongestants— Drugs used for a short term to reduce mucous membrane congestion.

Histamines— A chemical released from cells in the immune system as part of an allergic reaction.

IgE— The chief immunoglobulin responsible for producing the compounds that cause allergic reactions.

Lymphocyte— A white blood cell that stimulates antibody formation.

Mast cell— A tissue white blood cell located at the site of small blood vessels.

Receptor sites— Places on the mast cell where Y-shaped antibodies stimulate histamine production.

Rhinitis— The common condition of upper respiratory tract inflammation occurring in both colds and allergy.

Weal— The reddened, itchy swelling caused by a skin patch test.

muscle lining the airways to open or dilate, can be very effective. Some bronchodilators used to treat acute asthma attacks include adrenaline, albuterol, or other adrenoceptor stimulants, most often administered as aerosols. Theophylline, naturally present in coffee and tea, is another drug that produces brocho-dilation. It is usually taken orally, but in a severe asthma attack is may be given intravenously. Other drugs, including steroids, are used to prevent asthma attacks and in the long-term management of asthma.

Calamine lotion applied to affected skin can reduce irritation somewhat. Topical corticosteroid creams are more effective, though overuse may lead to dry and scaly skin.

The emergency condition of anaphylaxis is treated with injection of adrenaline, also known as epinephrine. People who are prone to anaphylaxis because of food or insect allergies often carry an Epi-pen containing adrenaline in a hypodermic needle. Prompt injection can prevent a more serious reaction from developing.

Allergies can improve over time, although they often worsen. While anaphylaxis and severe asthma are life threatening, other allergic reactions are not. Learning to recognize and avoid allergy-provoking situations allows most people with allergies to lead normal lives.

Avoiding allergens is the best means of limiting allergic reactions. For food allergies, there is no effective treatment except avoidance. By determining the allergens that are causing reactions, most people can learn to avoid allergic reactions from food, drugs, and contact allergens such as poison ivy. Airborne allergens are more difficult to avoid, although keeping dust and animal dander from collecting in the house may limit exposure.

See also Antibody and antigen.

Resources

BOOKS

Adelman, Daniel C., and Thomas B. Casale. Manual of Allergy and Immunology: Diagnosis and Therapy. 4th ed. Philadelphia: Lippincott Williams & Wilkins Publishers, 2002.

Holgate, Stephen T., Martin K. Church, and Lawrence M. Lichtenstein. Allergy. Philadelphia, PA: Mosby Elsevier, 2006.

Jackson, Mark. Allergy: The History of a Modern Malady. London, UK: Reaktion, 2006.

Wolf, Raoul L. Essential Pediatric Allergy, Asthma and Immunology. New York: McGraw-Hill Medical Pub. Division, 2004.

OTHER

American College of Allergy, Asthma & Immunology. “ACAAI Online.” <http://www.acaai.org/> (accessed November 27, 2006)

National Institutes of Health. “The National Institute of Allergy and Infectious Diseases” (November 6, 2006) >http://www.niaid.nih.gov/default.htm> (accessed November 27, 2006).

Jordan P. Richman

Allergies

Definition

Allergies are abnormal reactions of the immune system that occur in response to otherwise harmless substances.

Description

Allergies are among the most common medical disorders. It is estimated that 60 million Americans, or more than one in every five people, suffer from some form of allergy, with similar proportions throughout much of the rest of the world. Allergy is the single largest reason for school absence and is a major source of lost productivity in the workplace.

An allergy is a type of immune reaction. Normally, the immune system responds to foreign bodies, like pollen or bacteria, by producing specific proteins called antibodies that are capable of binding to identifying molecules (antigens) on the foreign body. This reaction between antibody and antigen sets off a series of reactions designed to protect the body from infection. Harmless, everyday substances trigger this same series of reactions. This is the condition known as allergy, and the offending substance is called an allergen.

Allergens enter the body through four main routes: the airways, the skin, the gastrointestinal tract, and the circulatory system.

• Airborne allergens cause the sneezing , runny nose, and itchy, bloodshot eyes of hay fever (allergic rhinitis ). Airborne allergens can also affect the lining of the lungs, causing asthma , or the conjunctiva of the eyes, causing conjunctivitis (pink eye).
• Allergens in food can cause itching and swelling of the lips and throat, cramps, and diarrhea . When absorbed into the bloodstream, they may cause hives or more severe reactions involving recurrent, non-inflammatory swelling of the skin, mucous membranes, organs, and brain (angioedema). Some food allergens may cause anaphylaxis, a potentially life-threatening condition marked by tissue swelling, airway constriction, and drop in blood pressure.
• In contact with the skin, allergens can cause reddening, itching, and blistering, called contact dermatitis . Skin reactions can also occur from allergens introduced through the airways or gastrointestinal tract. This type of reaction is known as atopic dermatitis .

• Injection of allergens, from insect bites and stings or drug administration, can introduce allergens directly into the circulation, where they may cause system-wide responses (including anaphylaxis), as well as the local responses like swelling and irritation at the injection site.

People with allergies are not equally sensitive to all allergens. Allergies may get worse over time. For example, childhood ragweed allergy may progress to year-round dust and pollen allergy. On the other hand, a person may lose allergic sensitivity. Infant or childhood atopic dermatitis disappears in almost all people. More commonly, what seems to be loss of sensitivity is instead a reduced exposure to allergens or an increased tolerance for the same level of symptoms.

Causes & symptoms

Causes

Immunologists separate allergic reactions into two main types: immediate hypersensitivity reactions, which are mainly mast cell-mediated and occur within minutes of contact with allergen, and delayed hypersensitivity reactions, mediated by T cells (a type of white blood cells) and occurring hours to days after exposure.

In the upper airways and eyes, immediate hypersensitivity reactions cause the runny nose and itchy, bloodshot eyes typical of allergic rhinitis. In the gastrointestinal tract, these reactions lead to swelling and irritation of the intestinal lining, which causes the cramping and diarrhea typical of food allergy. Allergens that enter the circulation may cause hives, angioedema, anaphylaxis, or atopic dermatitis.

Allergens on the skin usually cause delayed hypersensitivity reaction. Roving T cells contact the allergen, setting in motion a more prolonged immune response. This type of allergic response may develop over several days following contact with the allergen, and symptoms may persist for a week or more.

THE ROLE OF INHERITANCE. While allergy to specific allergens is not inherited, the likelihood of developing some type of allergy seems to be, at least for many people. If neither parent has allergies, the chances of a child developing allergy is approximately 10–20%; if one parent has allergies, it is 30–50%; and if both have allergies, it is 40–75%.

COMMON ALLERGENS. The most common airborne allergens are the following:

• plant pollens
• animal fur and dander
• body parts from house mites (microscopic creatures found in all houses)
• house dust
• mold spores
• cigarette smoke
• solvents
• cleaners

Common food allergens include the following:

• nuts, especially peanuts, walnuts, and Brazil nuts
• fish, mollusks, and shellfish
• eggs
• wheat
• milk
• food additives and preservatives

Common causes of contact dermatitis include the following:

• poison ivy, oak , and sumac
• nickel or nickel alloys
• latex

Insects and other arthropods whose bites or stings typically cause allergy include the following:

• bees, wasps, and hornets
• mosquitoes
• fleas
• scabies

Symptoms

Symptoms depend on the specific type of allergic reaction. Allergic rhinitis is characterized by an itchy, runny nose often with a scratchy or irritated throat due to postnasal drip. Inflammation of the thin membrane covering the eye (allergic conjunctivitis) causes redness, irritation and increased tearing in the eyes. Asthma causes wheezing , coughing, and shortness of breath. Symptoms of food allergies depend on the tissues most sensitive to the allergen and whether it is spread systemically by the circulatory system. Gastrointestinal symptoms may include swelling and tingling in the lips, tongue, palate or throat; nausea ; cramping; diarrhea; and gas . Contact dermatitis is marked by reddened, itchy, weepy skin blisters .

Whole body or systemic reactions may occur from any type of allergen, but are more common following ingestion or injection of an allergen. Skin reactions include the raised, reddened, and itchy patches called hives. A deeper and more extensive skin reaction, involving more extensive fluid collection, is called angioedema. Anaphylaxis, another reaction, is marked by difficulty breathing, blood pressure drop, widespread tissue swelling, heart rhythm abnormalities, lightheadedness, and in some cases, loss of consciousness.

Diagnosis

Allergies can often be diagnosed by a careful medical history, matching the onset of symptoms to the exposure to possible allergens. Allergy tests can be used to identify potential allergens. These tests usually begin with prick tests or patch tests, which expose the skin to small amounts of allergen to observe the response. Reaction will occur on the skin even if the allergen is normally encountered in food or in the airways. RAST testing is a blood test that measures the level of reactive antibodies in the blood. Provocation tests, most commonly done with airborne allergens, present the allergen directly through the route normally involved. Food allergen provocation tests require abstinence from the suspect allergen for two weeks or more, followed by ingestion of a measured amount. Provocation tests are not used if anaphylaxis is a concern due to the patient's medical history.

Treatment

Allergic rhinitis

The following can help to relieve the symptoms of airborne allergies:

• Stinging nettle (Urtica dioica ) has antihistamine and anti-inflammatory properties. The common dose is 300 mg four times daily.
• Grape (Vitis vinifera ) seed extract has antihistamine and anti-inflammatory properties. The usual dose is 50 mg three times daily.
• Ephedra (Ephedra sinicia ), also called ma huang, has anti-inflammatory activity and is proven effective in treating allergies. Taken as a tea thrice daily or 12.5–25 mg in capsule form. Ephedra should not be used for prolonged periods of time, as it can raise blood pressure, cause rapid heartbeat, and interfere with adrenal gland function.
• Licorice (Glycyrrhiza glabra ) has cortisone-like activity which is anti-inflammatory, stimulates the adrenals, and relieves allergy symptoms. Can be taken as a tea or 100–300 mg in capsule form. Long term use can result in sodium retention or potassium loss.
• Chinese skullcap (Scutellaria baicalensis ) has bronchodilator activity, is an anti-inflammatory, and prevents allergic reactions. It is taken in combination with other herbs.
• Ginkgo (Ginkgo biloba ) seeds are used in Chinese medicine for wheezing and coughing.
• Echinacea (Echinacea species) may have antiinflammatory activity and boost the immune system.
• Khellin (Ammi visnaga ) has bronchodilator activity.
• Cramp (Viburnum opulus ) bark has bronchodilator activity.
• Traditional Chinese medicine treats allergic rhinitis with various species. Patent combination medicines are: Bu Zhong Yi Qi Wan (Tonify the Middle and Augment the Qi) and Yu Ping Feng San (Jade Windscreen) are used for preventing allergies, and Bi Yan Pian (Rhinitis Infusion) is often prescribed for syptoms affecting the nose.
• The homeopathic remedies Rhus toxicodendron, Apis mellifica, and Nux vomica have decongestant activities. They are taken internally.
• Vitamin C has antihistamine and decongestant activities.
• Vitamins A and E are antioxidants and help to promote normal functioning of the immune system.
• Coenzyme Q ₁₀ may help to promote normal functioning of the immune system.
• Zinc may boost the immune system.
• N-acetylcysteine may have decongestant activity.
• Acupuncture has been shown to be as effective as antihistamine drugs in treating allergic rhinitis. It is also used to help prevent allergic reactions by strengthening the immune system.

Skin reactions

A variety of herbal remedies, either applied topically or taken internally, can assist in the treatment of contact dermatitis. A poultice made of jewelweed (Impatiens species) or chickweed (Stellaria media ) can soothe the skin. A cream or wash containing calendula (Calendula officinalis ), a natural antiseptic and anti-inflammatory agent, can help heal the rash. Chinese herbal remedies have been effective in treating atopic dermatitis. The following are homeopathic remedies to be taken internally:

• Apis (Apis mellifica ) for hives that feel better with cold application and bee stings
• Poison ivy (Rhus toxicodendron ) for hives that feel better with hot applications and for poison ivy, oak, or sumac rashes
• Stinging nettle (Urtica urens ) for hives
• Marsh tea (Ledum ) for itching insect bites
• Croton (Croton tiglium ) oil for poison ivy, oak, or sumac rashes

Food allergies

Food allergy may be managed by oral desensitization. Children with allergy to milk, eggs, fish, or apples who followed an oral desensitization procedure developed resistance to the allergenic food. Oral desensitization exposes the patient to allergens in controlled, but increasing, doses. Control subjects, who had avoided the allergenic food during the study, were still sensitive.

Allopathic treatment

A large number of prescription and over-the-counter drugs are available for treatment of immediate hypersensitivity reactions. Most of these drugs work by decreasing the ability of histamine to provoke symptoms. Other drugs counteract the effects of histamine by stimulating other systems or reducing immune responses in general.

ANTIHISTAMINES. Antihistamines block the histamine receptors on nasal tissue, decreasing the effect of histamine released by mast cells. They may be used after symptoms appear, though they seem to prove more effective when used preventively. A wide variety of antihistamines are available.

DECONGESTANTS. Decongestants constrict blood vessels to counteract the effects of histamine. Nasal sprays and oral systemic preparations are available. Decongestants are stimulants and may cause increased heart rate and blood pressure, headaches, and agitation. Use of nasal sprays for longer than several days can cause loss of effectiveness and rebound congestion, in which nasal passages become more severely swollen than before treatment.

TOPICAL CORTICOSTEROIDS. Topical corticosteroids reduce mucous membrane and skin inflammation and are available by prescription. Allergies tend to become worse as the season progresses and topical corticosteroids are especially effective at reducing this seasonal sensitization. As a result, they are best started before allergy season begins. A 2001 study revealed that steroid nasal sprays work better for seasonal allergies on an as-needed basis than do antihistamines. Side effects are usually mild, but may include headaches, nosebleeds , and unpleasant taste sensations.

MAST CELL STABILIZERS. Cromolyn sodium prevents the release of mast cell granules, thereby preventing the release of histamine and other chemicals contained in them. Cromolyn sodium is available as a nasal spray and aerosol (a suspension of particles in gas).

BRONCHODILATORS. Because allergic reactions involving the lungs cause the airways or bronchial tubes to narrow, bronchodilators, which cause the smooth muscle lining the airways to open, can be very effective. Bronchodilators include adrenaline, albuterol, and theophylline. Other drugs, including steroids, are used to prevent and control asthma attacks.

Immunotherapy

Immunotherapy, also known as desensitization or allergy shots, alters the balance of antibody types in the body. Injections involve gradually increasing amounts of allergen, over several weeks or months, with periodic boosters. Full benefits may take up to several years to achieve and are not seen at all in about one in five patients. Individuals receiving all shots will be monitored closely following each shot because of the small risk of anaphylaxis, a condition that can result in difficulty breathing and a sharp drop in blood pressure.

Possible future treatments

In late 2001, a reports stated that a monoclonal antibody called omalizumab might be effective in treating seasonal allergies and preventing related asthma. By blocking immunoglobulin E (IgE), an antibody that is found in excessive amounts in those with hay fever , the drug treats hay fever and helps prevent related asthma. Trials on the drug continue, and other immune-based medicines will likely accompany its ultimate release.

Treatment of contact dermatitis

Calamine lotion applied to affected skin can reduce irritation somewhat. Topical corticosteroid creams are more effective, though overuse may lead to dry and scaly skin.

Treatment of anaphylaxis

The emergency condition of anaphylaxis is treated with injection of adrenaline, also known as epinephrine. People who are prone to anaphylaxis because of food or insect allergies often carry an "Epi-pen" containing adrenaline in a hypodermic needle. Prompt injection can prevent a more serious reaction from developing.

Expected results

Allergies can improve over time, although they often worsen. While anaphylaxis and severe asthma are life-threatening, other allergic reactions are not. Learning to recognize and avoid allergy-provoking situations allows most people with allergies to lead normal lives.

Prevention

By determining which allergens are causing the reactions, most people can learn to avoid allergic reactions from food, drugs, and contact allergens. Airborne allergens are more difficult to avoid, although keeping dust and animal dander from collecting in the house may limit exposure. Vitamin C may prevent allergy symptoms. Cromolyn sodium can be used for allergy prevention.

Resources

BOOKS

Lawlor G.J., Jr, T.J. Fischer, and D.C. Adelman. Manual of Allergy and Immunology. Boston: Little, Brown and Co., 1995.

Weil, A. Natural Health, Natural Medicine: A Comprehensive Manual for Wellness and Self-Care. Boston: Houghton Mifflin, 1995.

Ying, Zhou Zhong and Jin Hui De. "Common Diseases of Otorhinolaryngology and Stomatology." In Clinical Manual of Chinese Herbal Medicine and Acupuncture. New York: Churchill Livingston, 1997.

PERIODICALS

Harder, Patty A. "Homeopathic First-Aid for Allergic Reactions." Better Nutrition. 60 (June 1998): 42.

Patriarca, G., D. Schiavino, E. Nucera, G. Schinco, A. Milani, and G.B. Gasbarrini. "Food Allergy in Children: Results of a Standardized Protocol for Oral Desensitization." Hepato-Gastroenterology. 45 (1998): 52-58.

Plaut, Marshall. "Immune-Based, Tageted Therapy for Allergic Diseases." JAMA, The Journal of the American Medical Association. (December 19, 2001): 3005.

Siegel-Maier, Karyn. "Nettlesome Allergies! Natural Alternatives to Treat Seasonal Sneezing." Mother Earth News. (September 1998): 24+.

"Steroid Nasal Spray Better for Season Allergies than Antihistamines." Immunotherapy Weekly (December 12, 2001): 12.

Belinda Rowland

Teresa Norris

Allergies

Definition

Allergies describe an immune response of the body to a substance that is not necessarily harmful in itself, but results in a reaction that causes symptoms and disease in a predisposed person. Description Allergies, also called hypersensitivity disorders, are triggered by an inappropriate response of the immune system, the body's defense mechanism against foreign substances. These include a wide variety of substances present in the environment, the air, and in foods. Common examples include pollen from trees and grasses, mold spores, pet dander, cockroach and dust mite droppings, and chemicals present in medicines or released by insects when they sting. Less common substances include nuts, fruit and latex. When they trigger an immune response, they are called allergens.

In a healthy person, antibodies circulate in the bloodstream and in almost all body fluids. They are produced by the immune system to help destroy foreign invaders. People who have allergies produce an antibody called Immunoglobulin E (IgE). When an allergen enters their body, IgE antibodies are produced against that allergen and attach to the surface of cells called mast cells, especially plentiful in the nose, eyes, lungs and gastrointestinal tract. A person may produce an IgE antibody against a specific allergen or several types of IgE against several different allergens. This is why some persons are only allergic to one allergen, for example to pollen, while people producing many types of IgE antibodies are allergic to many things.

When the IgE come into contact with an allergen, the mast cells release chemical substances called mediators (such as histamine) that cause the symptoms of an allergic response characterized by swelling of tissues, sneezing, wheezing, coughing and other reactions.

Common allergy diseases include:

• allergic conjunctivitis
• allergic dermatitis
• allergic reactions to food, latex, medications, and insect stings
• allergic skin reactions
• asthma
• hay fever (allergic rhinitis)
• hives (urticaria)

Demographics

Allergic disease is the fifth leading chronic disease in the United States among all ages. In 2005, a nationwide survey found that 54.6% of all Americans tested positive to one or more allergens. The incidence of allergic disease and asthma is increasing in industrialized countries. As of 2008, in the United Kingdom, allergy is widespread and affects approximately one in four of the population at some time in their lives. Each year the numbers are increasing by 5% with as many as half of all those affected being children. The American Academy of Allergy, Asthma and Immunology (AAAAI) reports that more than 50 million Americans had allergic diseases during the 1996–2001 period.

In 2003, the Center for Diseases Control (CDC) estimated the prevalence of hay fever in the United States at 9% to 16%, which represented a significant increase over previous years. Food allergies occur in 6–8% of children four years of age or under, and in 4% of adults. Approximately 12 million Americans have food allergy, with 6.9 million allergic to seafood. Approximately 150 Americans die every year from food-induced severe allergic reaction (anaphylaxis). Peanut or other nut allergies affect approximately 0.6–0.4% of Americans, causing the most severe food—induced allergic reactions. Allergic drug reactions account for 5–10% of all adverse drug reactions, with skin reactions being the most common symptom. Penicillin is a common cause of drug allergy, with an international study reporting in 2003 that anaphylactic reactions to penicillin occur in 32 of every 100,000 exposed patients. AAAAI reports that anaphylactic reactions to penicillin cause 400 deaths per year in the United States. Hives is a common allergy, affecting 10–20% of the population at some time in their lives. Half of those affected continue to have symptoms for more than six months. Allergy to the venom of stinging insects, such as honeybees, wasps, hornets, yellow jackets, and fire ants, occurs in 3% of Americans. Between 40 and 100 Americans have been reported to die annually from anaphylaxis to insect bites or stings. Estimates of the prevalence of allergy to latex allergens vary widely, from less than 1% to 6%. Approximately 220 cases of anaphylaxis and three deaths per year are due to latex allergy.

Causes and symptoms

Causes

Allergic reactions are caused by allergens. Most allergens contain proteins, organic compounds containing hydrogen, oxygen and nitrogen, which are the principal constituents of the cellular material of living organisms. There are some non-protein allergens, for example drugs such as penicillin, but to cause an allergic reaction they need to be bound to a protein once they are in the body. Allergens can get into the body through:

• inhalation into the nose and the lungs (airborne pollens; dust mite particles, mold spores, cat and dog dander and latex dust)
• ingestion by mouth as food (such as shrimp and peanuts)
• injection as medications (penicillin or other drugs)
• absorption through the skin (venom from insect stings and bites, substances from plants such as poison ivy)

The capacity to become allergic is an inherited characteristic and the genetic predisposition toward the development of hypersensitivity reactions upon exposure to specific antigens is called atopy. Medical research has shown that, after birth, the immune system switches to be either allergy prone (TH2) or non-allergy prone (TH1), depending on the interplay of genetics and environment. TH stands for T-helper white blood cells. TH1 cells are good for fighting bacteria and viruses, and protecting against allergies. TH2 cells are involved in fighting parasite infections, but make a person more likely to develop allergies. If there is a family history of allergies, a child is much more likely to switch on TH2 immunity, which promotes the manufacture of excessive amounts of IgE in the bloodstream that trigger allergic reactions.

Over the past four decades, atopy has significantly increased, but the reasons are poorly understood. Some families seem genetically prone to develop allergies, but several other factors are now believed to be involved. For instance, it has been suggested that allergy-inducing substances must have been added to the environment, while other factors that previously protected against allergies have probably been removed. There is also some evidence suggesting that the fight against infectious diseases and increased personal cleanliness may interfere with immune system function. Global warming has been reported as having an impact, with changing patterns of natural vegetation and increased pollen production.

Symptoms

Common symptoms of allergy include itching eyes, sneezing, nasal congestion, runny nose, severe wheezing, shortness of breath, and sometimes head-ache. Some people experience hearing changes, sore throats, hoarseness, and cough. Other less common symptoms are balance disturbances, swelling in face or throat tissues, skin irritations, and asthma. Some affected individuals experience allergic reactions all year, while others are only subjected to seasonal attacks. As a rule, inhaled allergens such as dust mites and tree, grass or weed pollens tend to induce respiratory symptoms while ingested allergens tend to produce skin and gastrointestinal symptoms or anaphylaxis. Both types of allergens can also produce the entire spectrum of allergy symptoms.

Diagnosis

Along with a patient's medical history, allergy tests are commonly used to confirm whether symptoms are caused by allergies. Allergy tests can determine which specific substances are triggering an allergic reaction and may include:

• Skin tests. These involve placing a tiny amount of suspected allergen onto the skin, which is then scratched to allow the allergen to penetrate under the skin. Afterwards, the skin is examined for signs of allergic reaction, such as redness or swelling.
• Elimination tests. These tests diagnose food allergies. The person is given a diet that avoids suspected allergenic foods for several weeks. After this period, each suspected food is reintroduced in the diet one at a time and the person is observed for signs of allergic reaction.
• Blood tests. These tests determine the amount of IgE in the blood.
• Provocation test. This type of test is performed only under close medical supervision and involves exposing a person to a suspected allergen under controlled circumstances.

Treatment

Treatment depends on the type of allergy. Food allergy is mostly treated by avoiding the foods that trigger the reaction. Medications in the form of pills, liquids, nasal sprays, eyedrops and skin creams are available to treat allergic symptoms. Some are available over-the-counter while others are available by prescription only. The appropriate medication depends on symptoms and overall health condition of the affected person. A physician may recommend trying a few different medications to determine which ones are most effective with the least side effects. Common allergy medications include:

• Antihistamines. These drugs block histamine, the inflammatory substance released from mast cells during an allergic reaction.
• Corticosteroids. This class of medications helps to prevent and treat the inflammation associated with allergic conditions.
• Cromolyn sodium. Another drug that blocks histamine.
• Decongestants. These medications relieve nasal and sinus congestion.
• Leukotriene modifiers. These drugs block the effects of leukotrienes, inflammatory substances released by the immune system during an allergic reaction.
• Injectable epinephrine. Epinephrine, also known as adrenalin, relaxes muscles and helps open the airways. People at risk of anaphylaxis because they are highly allergic to some foods, or to bee or wasp venom, can carry an single dose of epinephrine that is administered as a shot into the thigh to help slow the reaction until emergency medical treatment becomes available.

Nutrition/Dietetic concerns

People who have a food allergy must remove the food source from their diet, once it has been identified by a healthcare provider. Many allergy-inducing foods such as peanuts, eggs, and milk, are used as ingredients in other foodstuffs. Peanuts, for example, may be used as a protein source, and eggs are often used in some prepared foods and condiments such as salad dressings. People affected by food allergies must read the detailed ingredient lists and labels on all purchased food.

Therapy

People who have allergic symptoms that do not clear or improve with medications may be prescribed allergy shots, a form of immunotherapy in which affected persons receive regular injections that contain allergen extracts over a period of three to five years. The amounts of allergen injected are too small to trigger an allergic response, but large enough to stimulate the immune system. Over time, the amounts of allergen extract are increased and the body slowly becomes desensitized to specific allergens. Immunotherapy decreases or eliminates the need for medications. It is most effective for hay fever and insect sting allergies. While it may require years of treatment, it is effective in about two-thirds of cases.

QUESTIONS TO ASK YOUR DOCTOR

• What are the side effects of my prescribed medication?
• What foods are safe to eat?
• Should I consider immunotherapy?
• What are the respective advantages and disadvantages?
• How does my medication work?

Prognosis

Most allergies are successfully treated with medication. However, medications can only relieve allergic symptoms and cannot prevent future allergic reactions.

Children younger than three years who are diagnosed with anaphylaxis to foods such as milk, eggs, wheat, or soybeans often outgrow such allergies after several years. Children who develop food sensitivity after three years of age are less likely to become free of allergic reactions. Allergies to foods such as tree nuts, fish, and seafood are generally not outgrown regardless of the age at which they develop. These persons are likely to retain their allergic sensitivity for a lifetime.

Prevention

The Asthma and Allergy Foundation of America provides the following guidelines to prevent allergies:

• dust homes to control mites
• vacuum often
• reduce pet dander
• prevent pollen from entering the home by keeping windows and doors closed
• eliminate mold spores

Caregiver concerns

Even though the incidence of allergies is increasing worldwide, there is a lack of research on the incidence and prevalence of allergic sensitization in the nonpediatric population even though studies suggest that allergies in elderly people are not rare. The elderly population is estimated to increase by 75% between 2010 and 2030. It is accordingly expected that a growing proportion of allergy patients will be from this age group, and it will become increasingly important to properly diagnose allergy, especially in patients with rather mild symptoms, such as conjunctivitis or rhinitis. Although asthma can be associated with significant airflow obstruction in those over the age of 65, it is often underdiagnosed and undertreated. Some studies on the use of topical medications by the elderly have noted allergic positive patch tests in between 43% and 64% of those tested, showing that this type of allergy is more common in those aged more than 70 years when compared with younger age groups.

KEY TERMS

Allergen —Any substance that causes an allergic response. Allergens contain protein, and almost anything can be an allergen for someone.

Allergenic —Acting as an allergen, inducing allergy.

Allergic conjunctivitis —Inflammation of the conjunctiva, the membrane covering the white part of the eye, due to allergy.

Allergic dermatitis —Itching, reddening, and flaking or peeling of the skin resulting from allergen exposure to the skin.

Allergic rhinitis —An allergy affecting the mucus membrane of the nose. Seasonal allergic rhinitis is called hay fever.

Anaphylaxis —An acute, severe, life-threatening allergic reaction.

Antibody —A protein produced by the body's immune system in response to a foreign substance.

Antigen —Any foreign substance, usually a protein, that stimulates the body's immune system to produce antibodies.

Asthma —Respiratory disorder characterized by wheezing; usually of allergic origin.

Atopy —Genetic predisposition toward the development of hypersensitivity reactions, such as hay fever, asthma, or hives, upon exposure to specific antigens.

Epinephrine —Hormone released into the bloodstream in response to stress, as from fear or injury. It initiates many bodily responses, including the stimulation of heart action and an increase in blood pressure, metabolic rate, and blood glucose concentration.

Heparin —A substance found especially in lung and liver tissue that can prevent the clotting of blood.

Histamine —Substance released from mast cells during an allergic reaction. It causes itching, sneezing, increased mucous production, and nasal congestion.

Hives —A raised, itchy area of skin that is usually a sign of an allergic reaction.

Hypersensitivity —An exaggerated response by the immune system to a substance.

Immune —Resistant to a given disease.

Immune response —The activity of the immune system against antigens.

Immunoglobulin E (IgE) —Antibodies produced in the lungs, skin, and mucous membranes that are responsible for allergic reactions.

Immunotherapy —Treatment to stimulate or restore the ability of the immune system to fight disease.

Mast cells —A cell found in connective tissue that contains granules that release substances such as heparin and histamine during allergic reactions.

Resources

BOOKS

Brooks, Pamela. The “Daily Telegraph” Complete Guide to Allergies. London, UK: Robinson Publishing, 2001.

Brostoff, Jonathan, and Linda Gamlin. Food Allergies and Food Intolerance: The Complete Guide to Their Identification and Treatment. Rochester, VT: Healing Arts Press, 2000.

Hicks, Rob. Beat Your Allergies: Find Relief, Feel Free. Oxford, UK: The Infinite Ideas Company, 2005.

Kwong, Frank. The Complete Allergy Book: Learn to Become Actively Involved in Your Own Care. Naperville, IL: Sourcebooks, Inc., 2002.

Miner, Margaret, and Stuart H. Young. Allergies: The Complete Guide to Diagnosis, Treatment, and Daily Management. New York, NY: Penguin Books, 1999.

Mitman, Gregg. Breathing Space: How Allergies Shape Our Lives and Landscapes. New Haven, CT: Yale University Press, 2007.

Pescatore, Fred. The Allergy and Asthma Cure: A Complete Eight-Step Nutritional Program. New York, NY: Wiley, 2003.

Wright, Tanya. Food Allergies. London, UK: Class Publishing, 2006.

Zellerbach, Merla. The Allergy Sourcebook. New York, NY: McGraw-Hill, 2000.

PERIODICALS

Bakos, N., et al. “Risk Assessment in Elderly for Sensitization to Food and Respiratory Allergens. “Immunology Letters 107, no. 1 (September 2006): 15–21.

Bellanti, J. A., et al. “Possible Mechanisms of Late-life-onset Allergic Diseases and Asthma in the Senior Citizen. “Allergy and Asthma Proceedings 21 (2000): 267–270.

Green, C.M., C. R. Holden, and D. J. Gawkrodger. “Contact Allergy to Topical Medicaments Becomes More Common with Advancing Age: An Age-stratified Study. “Contact Dermatitis 56, no. 4 (April 2007): 229–231.

Huss, K., et al. “Asthma Severity, Atopic Status, Allergen Exposure and Quality of Life in Elderly Persons. “Annals of Allergy Asthma and Immunology 8 (2001): 524–530.

Wöhrl, S., and G. Stingl. “Underestimation of Allergies in Elderly Patients. “Lancet 363, no. 9404 (January 2004): 249.

OTHER

Airborne Allergens: Something in the Air. NIAID Fact Sheet. NIH Publication No. 03-7045. April 2003 [cited April 9, 2008]. http://www3.niaid.nih.gov/healthscience/healthtopics/allergicDiseases/PDF/airborne_allergens.pdf.

Gardening with Allergies. AAFA Fact Sheet. (February 20, 2008) http://www.aafa.org/display.cfm?id=9&sub=19&cont=470

Pauls, John D. Seniors and Asthma: Getting the Medication and Dosage Right. AAAAI. June 2007 [cited April 9, 2008]. http://www.aaaai.org/patients/seniorsandasthma/medications_and_dosage.stm.

Tips to Remember: What is an Allergic Reaction? AAAAI Fact Sheet. 2007 [cited April 9, 2008]. http://www.aaaai.org/patients/publicedmat/tips/whatisallergicreaction.stm

ORGANIZATIONS

American Academy of Allergy, Asthma & Immunology (AAAAI), 555 East Wells St., Milwaukee, WI, 53202–3823, (414) 272-6071, info@aaaai.org, http://www.aaaai.org.

Asthma and Allergy Foundation of America (AAFA), 1233 20th Street, NW, Suite 402, Washington, DC, 20036, (202) 466-7643, (800) 7-ASTHMA, Info@aafa.org, http://www.aafa.org.

National Institute of Allergy and Infectious Diseases (NIAID), 6610 Rockledge Drive, MSC 6612, Bethesda, MD, 20892-6612, (301) 496-5717, (866) 284-4107, http://www3.niaid.nih.gov.

Monique Laberge Ph.D.