The Lymphatic System

The lymphatic system is often considered part of the cardiovascular system (see chapter 1). Excess fluid that leaks out of capillaries to bathe the body's cells is collected by the vessels of the lymphatic system and returned to the blood. By doing so, the lymphatic system maintains the fluid balance in the body. The lymphatic system further assists the cardiovascular system in absorbing nutrients from the small intestine. These necessary actions, however, are only part of the system's vitally important overall function. It is the body's main line of defense against foreign invaders such as bacteria and viruses. The lymphatic system is responsible for body immunity, filtering harmful substances out of tissue fluid (which fills the spaces between the cells) before that fluid is returned to the blood and the rest of the body. For this reason, it is sometimes referred to as the immune system.

DESIGN: PARTS OF THE LYMPHATIC SYSTEM

A network of vessels, tissues, organs, and cells constitute the lymphatic system. Included in this network are lymph vessels, lymph nodes, the spleen, the thymus, and lymphocytes. Running throughout this network is a watery fluid called lymph.

Lymph

Lymph comes from the Latin word lympha, meaning "clear water." Slightly yellowish but clear, lymph is any tissue or interstitial fluid that enters the lymph vessels. It is similar to blood plasma, but contains more white blood cells. Lymph also carries other substances, depending on where it is in the body. In the limbs, lymph is rich in protein, especially albumin. In the bone marrow, spleen, and thymus, lymph contains higher concentrations of white blood cells. And in the intestine, lymph contains fats absorbed during digestion.

Lymph vessels

Lymph vessels, also called lymphatics, carry lymph in only one direction—to the heart. Throughout all the tissues of the body, lymph vessels form a complicated, spidery network of fine tubes. The smallest vessels, called lymph capillaries, have closed or dead ends (unlike vessels in the cardiovascular system, which form a closed circuit). The walls of these capillaries are composed of only a single layer of flattened cells. Material in the interstitial fluid passes easily through the gaps between these cells into the capillaries. Lymph capillaries in the villi (tiny fingerlike projections) of the small intestine are called lacteals. These specialized capillaries transport the fat products of digestion, such as fatty acids and vitamin A.

The Lymphatic System: Words to Know

Allergen (AL-er-jen):

Substance that causes an allergy.

Antibody (AN-ti-bod-ee):

Specialized substance produced by the body that can provide immunity against a specific antigen.

Antibody-mediated immunity (AN-ti-bod-ee MEE-deea-ted i-MYOO-ni-tee):

Immune response involving B cells and their production of antibodies.

Antigen (AN-ti-jen):

Any substance that, when introduced to the body, is recognized as foreign and activates an immune response.

B cell:

Also called B lymphocyte, a type of lymphocyte that originates from the bone marrow and that changes into antibody-producing plasma cells.

Cell-mediated immunity (CELL MEE-dee-a-ted i-MYOO-ni-tee):

Immune response led by T cells that does not involve the production of antibodies.

Chyle (KILE):

Thick, whitish liquid consisting of lymph and tiny fat globules absorbed from the small intestine during digestion.

Edema (i-DEE-mah):

Condition in which excessive fluid collects in bodily tissue and causes swelling.

Fever:

Abnormally high body temperature brought about as a response to infection or severe physical injury.

Histamine (HISS-ta-mean):

Chemical compound released by injured cells that causes local blood vessels to enlarge.

Immunity (i-MYOO-ni-tee):

Body's ability to defend itself against pathogens or other foreign material.

Inflammation (in-flah-MAY-shun):

Response to injury or infection of body tissues, marked by redness, heat, swelling, and pain.

Interferon (in-ter-FIR-on):

Protein compound released by cells infected with a virus to prevent that virus from reproducing in nearby normal cells.

Lacteals (LAK-tee-als):

Specialized lymph capillaries in the villi of the small intestine.

Lymph (LIMF):

Slightly yellowish but clear fluid found within lymph vessels.

Lymph node:

Small mass of lymphatic tissue located along the pathway of a lymph vessel that filters out harmful microorganisms.

Lymphocyte (LIM-foe-site):

Type of white blood cell produced in lymph nodes, bone marrow, and the spleen that defends the body against infection by producing antibodies.

Macrophage (MACK-row-fage):

Large white blood cell that engulfs and destroys bacteria, viruses, and other foreign substances in the lymph.

Natural killer cell:

Also known as an NK cell, a type of lymphocyte that patrols the body and destroys foreign or abnormal cells.

Peyer's patches (PIE-erz):

Masses of lymphatic tissue located in the villi of the small intestine.

Phagocyte (FAG-oh-site):

Type of white blood cell capable of engulfing and digesting particles or cells harmful to the body.

Phagocytosis (fag-oh-sigh-TOE-sis):

Process by which a phagocyte engulfs and destroys particles or cells harmful to the body.

Spleen:

Lymphoid organ located in the upper left part of the abdomen that stores blood, destroys old red blood cells, and filters pathogens from the blood.

T cell:

Also known as T lymphocyte, a type of lymphocyte that matures in the thymus and that attacks any foreign substance in the body.

Thoracic duct (tho-RAS-ik):

Main lymph vessel in the body, which transports lymph from the lower half and upper left part of the body.

Thymus (THIGH-mus):

Glandular organ consisting of lymphoid tissue located behind the top of the breastbone that produces specialized lymphocytes; reaches maximum development in early childhood and is almost absent in adults.

Tonsils (TAHN-sills):

Three pairs of small, oval masses of lymphatic tissue located on either side of the inner wall of the throat, near the rear openings of the nasal cavity, and near the base of the tongue.

Vaccine (vack-SEEN):

Substance made of weakened or killed bacteria or viruses injected (or taken orally) into the body to stimulate the production of antibodies specific to that particular infectious disease.

Villi (VILL-eye):

Tiny, fingerlike projections on the inner lining of the small intestine that increase the rate of nutrient absorption by greatly increasing the intestine's surface area.

As lymph capillaries carry lymph away from the tissue spaces, they merge to form larger and larger vessels. These larger lymph vessels resemble veins, but their walls are thinner and they have more one-way valves to prevent lymph from flowing backward. Whereas the cardiovascular system has a pump (the heart) to move fluid (blood) through the system, the lymphatic system does not. It relies on the contraction of muscles to move lymph throughout the body. The larger lymph vessels have a layer of smooth muscle in their walls that contracts rhythmically to "pump" lymph along. The contraction of skeletal muscles, brought about by simple body movement, and the mechanics of breathing also help to move lymph on its way.

The successively larger lymph vessels eventually unite to return lymph to the venous system through two ducts or passageways: the right lymphatic duct and the thoracic duct. Lymph that has been collected from the right arm and the right side of the head, neck, and thorax (area of the body between the neck and the abdomen) empties into the right lymphatic duct. Lymph from the rest of the body drains into the thoracic duct, the body's main lymph vessel, which runs upward in front of the backbone.

Both ducts then empty the lymph into the subclavian vein, which lies under the clavicle or collarbone. The right lymphatic duct empties into the right subclavian vein; the thoracic duct empties into the left subclavian vein. Flaps in both subclavian veins allow the lymph to flow into the veins, but prevent

it from flowing backward into the ducts. The subclavian veins empty into the superior vena cava, which then empties into the right atrium of the heart.

Lymph nodes

Scattered along the pathways of lymph vessels are oval or kidney bean-shaped masses of lymphatic tissue called lymph nodes, which act as filters. These nodes range in size from microscopic to just under 1 inch (2.5 centimeters) in length. The smaller lymph nodes are often called lymph nodules.

Between 500 and 1,500 lymph nodes are located in the body; most of them usually occur in clusters or chains. Principal groupings are based in the neck, armpits, chest, abdomen, pelvis, and groin. The lymph nodes in the neck, armpits, and groin are especially important because they are located where the head, arms, and legs (the extremities) meet the main part of the body (the trunk). Most injuries to the skin, which allow bacteria and other pathogens (disease-causing organisms) to enter the body, are likely to occur along the extremities. The lymph nodes at the junctions of the extremities and trunk destroy the pathogens before they reach the main part of the body and the vital organs.

THE BLACK DEATH

In 1347, several Italian merchant ships returned to Messina on the Mediterranean island of Sicily from a trip to the Black Sea. As the ships were docking, many sailors on board were dying of a strange and hideous disease. Within days, many residents of Messina and the surrounding countryside had been infected and were dying. Within four years, the disease had spread across Western Europe and 25 million people—roughly one-third the population of Europe at the time—lay dead in its wake.

The disease was called the "Black Death" because of the black spots it produced on the skin of its victims. It is more properly known as the bubonic plague (a plague is any contagious, widespread disease). In the course of recorded history, a number of major bubonic plagues have swept across Asia and Europe. The worst, however, was the outbreak in the fourteenth century. Because so many people died, there were serious labor shortages throughout Europe. Economic growth on the continent was halted for two centuries.

Historians believe this particular plague started in China sometime in the early 1330s. As one of the world's busiest trading nations at the time, China was a prime destination for the merchant ships of Europe. But those ships brought back to the West something no one bargained for.

Bubonic plague is an infectious disease caused by Yersinia pestis, a bacteria transmitted by fleas that have fed on the blood of infected rodents, usually rats. The fleas pass on the bacteria, in turn, when they bite a human. Humans may also become infected if they have a cut or break in the skin and come in direct contact with the body fluids or tissues of infected animals or other humans.

Once inside the human body, Yersinia pestis acts quickly. Within two to five days, victims experience fever, headache, nausea, vomiting, and aching joints. The lymph nodes, especially those in the groin, then suddenly become painful and swollen. The inflamed nodes, called buboes (from which the disease gets its name), swell with pus to about the size of a chicken's egg. They soon turn black, split open, and ooze pus and blood onto the skin. The infection, rampant in the lymphatic system, quickly spreads throughout the body. Blood appears in the victim's urine and stool, and everything coming out of the body smells horrible. When death finally comes less than a week after the victim contracts the disease, it is painful.

Isolated cases of bubonic plague continue to occur in widespread regions of the world. Since the disease is caused by a bacteria, it can be treated with antibiotics. If treated quickly, the plague can almost always be cured.

Each lymph node is enclosed in a fibrous capsule. Lymph enters the node through several small lymph vessels. Inside, bands of connective tissue divide the node into spaces known as sinuses. The specialized tissue in these sinuses harbors macrophages and lymphocytes, both of which are types of white blood cells. Macrophages engulf and destroy bacteria and other foreign substances in the lymph. Lymphocytes identify foreign substances and attempt to destroy them, also. If foreign invaders are abundant and macrophages and lymphocytes have to increase in number to defend the body against them, the lymph node often becomes swollen and tender. Once the lymph has been filtered and cleansed, it leaves the node through one or two other small lymph vessels.

Tonsils and Peyer's patches

Both tonsils and Peyer's patches are small masses of lymphatic tissue (some sources consider them specialized lymph nodes). These tissues serve to prevent infection in the body at areas where bacteria is abundant. There are five tonsils: a pair on either side of the inner wall of the throat (palatine tonsils), one near the rear opening of the nasal cavity (pharyngeal tonsil), and a pair near the base of the tongue (lingual tonsils). This "ring" around the throat helps trap and remove any bacteria or other foreign pathogens entering the throat through breathing, eating, or drinking. Peyer's patches, which resemble tonsils, are located in the small intestine. The macrophages of Peyer's patches prevent infection of the intestinal wall by destroying the bacteria always present in the moist environment of the intestine.

The spleen

The spleen is a soft, dark purple, bean-shaped organ located in the upper left side of the abdomen, just behind the bottom of the rib cage. It is the largest mass of lymphoid tissue in the body, measuring about 5 inches (12.7 centimeters) in length.

Though considered to be part of the lymphatic system, the spleen does not filter lymph (only lymph nodes do so). Instead, it filters and cleanses blood of bacteria, viruses, and other pathogens. It also destroys worn or old red blood cells. As blood flows through the spleen, macrophages lining the organ's tissues engulf and destroy both pathogens and worn red blood cells. Any remaining parts of decomposed red blood cells, such as iron, are returned to the body to be used again to form new red blood cells.

Other functions of the spleen include the production of lymphocytes, which the organ releases into the bloodstream, and blood storage. When the body demands additional blood, such as during stress or injury, the spleen contracts, forcing its stored blood into circulation.

The thymus

The thymus is a soft, flattened, pinkish-gray mass of lymphoid tissue located in the upper chest under the breastbone. In a fetus and newborn infant, the thymus is relatively large (about the size of an infant's fist). Up until about the age of puberty, the thymus continues to grow. After this point in life, it shrinks and gradually blends in with the surrounding tissue. Very little thymus tissue is found in adults.

Scientists did not fully understand the function of the thymus until the early 1960s. Only then was its role in developing body immunity discovered.

In a fetus and infant, immature or not fully developed lymphocytes are produced in the bone marrow (the spongelike material that fills the cavities inside most bones). A certain group or class of these lymphocytes travel to the thymus where thymic hormones changed them into T lymphocytes or T cells (the letter T refers to the thymus). While maturing and multiplying in the thymus, T cells are "educated" to recognize the difference between cells that belong to the body ("self") and those that are foreign ("nonself"). Each T cell is programmed to respond to a specific chemical identification marker—called an antigen—on the surface of foreign or abnormal cells. Once they are fully mature, T cells then enter the bloodstream and circulate to the spleen, lymph nodes, and other lymphatic tissue.

Lymphocytes

Lymphocytes, the primary cells of the lymphatic system, make up roughly one-fourth of all white blood cells in the body. Like other white blood cells, they are produced in the red bone marrow. Lymphocytes constantly travel throughout the body, moving through tissues or through the blood or lymph vessels.

There are two major classes of lymphocytes: T cells and B cells. As already stated, the letter T refers to the thymus, where those lymphocytes mature. The letter B refers to the bone marrow, where that group of lymphocytes mature.

About three-quarters of the circulating lymphocytes are T cells. They carry out two main defensive functions: they kill invaders and orchestrate or control the actions of other lymphocytes involved in the immune process or response. In addition, T cells recognize and destroy any abnormal body cells, such as those that have become cancerous.

Like T cells, B cells are also programmed to recognize specific antigens on foreign cells. When stimulated during an immune response (such as when foreign cells enter the body), B cells undergo a change in structure. They then produce antibodies, which are protein compounds. These compounds bind with specific antigens of foreign cells, labeling those cells for destruction.

WORKINGS: HOW THE LYMPHATIC SYSTEM FUNCTIONS

The exchange of materials (oxygen, carbon dioxide, nutrients, and wastes) between the blood and the cells in the body occurs through the capillaries. In the body of an average person, over the course of an average day, roughly 25.4 quarts (24 liters) of plasma fluid are forced out of the capillaries into the interstitial fluid surrounding the cells. After bathing the cells, providing them with nutrients, and picking up their wastes, this fluid is drawn back into the capillaries. However, only 85 percent of the total fluid is drawn back into the bloodstream. The remaining 15 percent, roughly 3.8 quarts (3.6 liters), remains in the interstitial fluid.

If this small amount of fluid were allowed to accumulate over even a brief period of time, massive edema (swelling caused by excessive bodily fluid) would result. If left unchecked, the body would blow up like a balloon, tissues would be destroyed, and death would take place. This condition is prevented by the presence of lymph capillaries, which run alongside blood vessels in most tissue spaces. The lymph capillaries act as "drains," collecting the excess fluid and returning it to the venous blood just before the blood reaches the heart.

Proteins and other large molecules dissolved in the interstitial fluid cannot be absorbed by the blood capillaries. Because the walls of lymph capillaries are much more permeable (allow material to pass through easily), these large substances enter the lymph capillaries and are eventually returned to the blood.

This function of lymph capillaries is particularly important in the small intestine. Whereas carbohydrates and certain other nutrients are small enough to pass directly from the intestine into the bloodstream, fats are not. Lacteals (lymph capillaries in the small intestine) are able to absorb fats and other nutrients that are too large to enter blood capillaries. After digestion, the lymph in lacteals contains as much as 1 to 2 percent fat. Milky-white in appearance, this thick mixture of lymph and tiny fat globules is called chyle. It becomes mixed with the blood after lymph drains into the thoracic duct.

After the vessels of the lymphatic system have collected excess fluid, cellular wastes, proteins, fats, and other substances too large to enter the blood capillaries directly, they return all this material to the bloodstream. Before advancing to the heart and reentering the circulatory system, however, any fluid and matter that enters the lymphatic system must pass through at least one lymph node. It is very likely that foreign substances, such as viruses, bacteria, and even cancer cells, are a part of the lymph that has been collected from all parts of the body. Even the dark, gritty debris of polluted city air finds its way from the lungs of city dwellers into the lymph. Without the filtering abilities of lymph nodes, these foreign substances would overrun the body.

SABIN AND THE LYMPHATIC SYSTEM

Florence Rena Sabin, a pioneering medical researcher and educator, helped settle a longstanding controversy: how the lymphatic system developed in the human body. In a field dominated by men, Sabin achieved much: she was the first woman faculty member at Johns Hopkins School of Medicine (as well as its first female professor) and the first woman elected to membership in the National Academy of Sciences.

Born in Central City, Colorado, in 1871, Sabin graduated from Smith College in Massachusetts with a bachelor of science degree in 1893. Three years later, she entered Johns Hopkins School of Medicine. After graduating in 1900, Sabin was accepted as an intern at Johns Hopkins Hospital, a rare occurrence for a woman at the time. But her interest lay in research and teaching, and she was soon awarded a fellowship in the department of anatomy at Johns Hopkins School of Medicine.

In this position, Sabin began her studies of the lymphatic system. At the time, researchers were split over whether the lymphatic system developed separately from the vessels of the cardiovascular system. To find the definitive answer, Sabin studied pig embryos because they developed in a manner similar to humans. (An embryo is an organism in its earliest stages of development.)

After painstaking research, Sabin discovered that lymphatic vessels did, in fact, arise from veins. She found that the outer layer of cells on veins sprouted buds, much like stems growing out of the branches of trees. As these stems grew outward, they connected with each other. This proved that the lymphatic system developed entirely from existing vessels in the body. For her work on lymphatics, Sabin won the 1903 prize of the Naples Table Association, an organization that maintained a research position for women at the Zoological Station in Naples, Italy.

Throughout her career, Sabin continued to study the lymphatic system and the body's immune responses. She paid special attention to the bacterium that causes tuberculosis, an infectious disease marked by lesions on the lungs, bones, and other body tissues.

Sabin died in Denver, Colorado, in 1953.

Immunity is the body's ability to defend itself against pathogens or other foreign material. The human body is awash in a sea of infectious agents such as bacteria, viruses, fungi, and parasites. It provides an ideal habitat for many of them. If these organisms were to break through the barriers erected by the body, they would ultimately destroy it. To protect itself, the human body relies on two lines of defense: nonspecific and specific.

The nonspecific defense system

The nonspecific defense system prevents the entry and spread of foreign microorganisms throughout the body. It is a general defense that does not discriminate between one threat and another; it responds to protect the body from all foreign substances, whatever they are. Present at birth, it includes physical barriers and cellular/chemical defenses.

PHYSICAL BARRIERS. Physical barriers are the body's first line of defense against disease-causing microorganisms. These barriers include the skin and mucous membranes. The skin, the outer tissue covering of the body, forms a strong physical barrier to most microorganisms that swarm on its surface. Microorganisms are unable to penetrate the upper layers of dead skin cells. In addition, the secretions from sweat and sebaceous (oil) glands in the skin contain chemicals that inhibit the growth of bacteria.

The membranes that line all body cavities open to the exterior—digestive, respiratory, urinary, and reproductive tracts—secrete or release sticky mucus that traps microorganisms. To further trap or destroy foreign invaders, the membranes in some of these tracts also contain hairs or secrete a variety of protective chemicals.

For instance, nasal hairs in nasal passages filter and trap large microorganisms before they enter the trachea or windpipe. Those invaders that do get by are often caught in the cilia, the microscopic hairlike structures projecting from the mucous membrane lining the trachea. The cilia wave back and forth in rhythmic movement, and trapped particles are swept along up through the trachea and into the throat. From here, the mucus and particles are either expelled by sneezing or coughing or swallowed into the stomach.

The mucous membrane of the stomach secretes hydrochloric acid and protein-digesting enzymes. Both kill microorganisms. Bacteria is also destroyed by saliva in the mouth and tears in the eyes, which each contain an enzyme that breaks down the walls of bacteria cells.

PHAGOCYTES. If a microorganism is able to break through the body's surface barriers, such as through a scrape or cut, the body uses an enormous number of cells and chemicals to protect itself. Any bacteria or foreign material that enters the body is immediately confronted by phagocytes. These are types of white blood cells that destroy foreign particles by surrounding them and engulfing them—a process called phagocytosis. The largest group of phagocytes are macrophages (literally, "big eaters"). A macrophage, which arises from a white blood cell called a monocyte, can "eat" up to 100 bacteria.

Another group of cells in the nonspecific body defense line are natural killer or NK cells. These type of lymphocytes are a unique group of defensive cells. They constantly patrol the body, floating in blood and lymph, seeking out antigens on foreign or abnormal body cells (such as those infected with viruses or cancer). NK cells can react against any such cells—they are not specific. Unlike macrophages, however, they are not phagocytic. When confronted with a foreign or abnormal cell, an NK cell secretes proteins that dissolve the target cell's membrane and the cell quickly disintegrates.

INFLAMMATION. Inflammation is the body's second line of defense. It develops whenever body tissues are damaged by physical injury or infected by bacteria and viruses. Inflammation is localized, meaning it occurs only where the injury or infection has taken place. The four major symptoms marking inflammation are redness, heat, swelling, and pain.

The inflammation process begins when damaged cells release chemicals such as histamine into the surrounding interstitial fluid. Histamine and the other chemicals cause the local blood vessels to expand, which increases blood flow to the area (accounting for the redness and heat). This, in turn, brings more oxygen, nutrients, and white blood cells into the area to fight the infection and begin the healing process. The heat produced by the additional blood increases the rate at which nearby cells "work" and creates unfavorable conditions for bacterial growth. As the blood vessels expand, they also become more permeable, allowing plasma from the bloodstream to seep into the interstitial spaces. This causes edema or swelling, which then activates local pain receptors. Both swelling and pain restrict movement in the area, further aiding healing.

FEVER. The body also uses heat in a more general way to fight infection: fever. A fever is a continued overall body temperature greater than 99°F (37°C). The hypothalamus is a portion of the brain that controls many body functions, including body temperature. In a sense, the hypothalamus is the body's thermostat. When white blood cells and macrophages are exposed to bacteria and other foreign invaders, they secrete chemicals that signal the hypothalamus to raise body temperature. The added heat in the body deters the growth of bacteria and speeds up the repair processes in the damaged cells. However, the temperature range at which fevers are beneficial is limited. High fevers—with temperatures over 104°F (40°C)—can damage many organ systems.

INTERFERONS. In 1957, scientists discovered another nonspecific defense mechanism: small protein compounds called interferons. Once infected with a virus, damaged cells help defend nearby uninfected cells by releasing interferons. These proteins travel to normal cells and bind to their membranes. Once attached, the interferons cause the normal cells to produce substances that prevent the virus from reproducing within those cells.

The specific defense system

The specific defense system is sometimes referred to as the body's third line of defense. Like the nonspecific defense system, it seeks out foreign invaders and acts to disable or destroy them. Unlike its sister system, however, the specific defense system recognizes and acts against particular foreign invaders or abnormal body cells. It is not limited in its actions to the initial site of infection, but functions throughout the entire body. It also has a "memory," mounting a quicker and stronger attack against viruses and bacteria when it encounters them a second time. For all these reasons, the specific defense system is much more effective in defending the body and establishing immunity.

The two separate but overlapping classes of lymphocytes—B cells and T cells—lead the specific immune response. The class called into play depends on the type of foreign or infected cells present in the body. B cells defend against invading bacteria and viruses. Because B cells produce antibodies to attack specific antigens, the immune response launched by B cells is called antibody-mediated immunity. T cells also defend the body against viruses, parasites, fungi, and other invaders. In addition, they attack body cells infected by viruses and bacteria and those that are cancerous. Because T cells attack pathogens directly, their immune response is called cell-mediated immunity.

T CELLS AND IMMUNITY. Cell-mediated immunity is triggered when a macrophage encounters a foreign or infected cell. The macrophage engulfs the cell, breaks it down, then "displays" fragments of that cell's antigens (chemical identification markers) on the outer surface of its membrane. The macrophage then "presents" itself to a T cell that been trained to recognize those specific antigens as nonself or foreign. Once this occurs, the T cell becomes activated and divides into four types: helper T cells, killer (or cytotoxic) T cells, suppressor T cells, and memory T cells. These T cells then travel to the point of infection in the body.

Helper T cells direct or manage the immune response, not only at the site of infection but throughout the body. They stimulate the production of more T cells and antibodies by B cells. They also attract other types of protective white blood cells into the infected area and spur the actions of killer T cells. As their name suggests, killer T cells are responsible for the destruction of foreign or abnormal cells. They kill a target cell by binding with it and injecting a chemical into the target cell, rupturing its membrane.

Once the invading or abnormal cells have been destroyed and the infection has been brought under control, suppressor T cells take over. They release chemicals that slow down and eventually stop the action of other T cells and B cells. This prevents the immune response from remaining active when it is not necessary and possibly harming normal body cells.

Most of the T cells gathered to fight infected or abnormal body cells die after only a few days. Those that remain are called memory T cells. Their job is to "remember" the antigens of each invader or abnormal cell so that if they appear again in the future, the body will be able to respond quickly and efficiently. Memory T cells are long-lived, often surviving in the body for up to twenty years or more.

LEADING THE FIGHT AGAINST AIDS

Since 1981, Anthony S. Fauci has play a significant part in AIDS research in the United States. As director of the National Institute of Allergy and Infectious Diseases (NIAID) and the Office of AIDS Research at the National Institutes of Health (NIH), Fauci supervises the ongoing investigation of how the disease works and the development of vaccines and drugs to treat and cure it.

Born in 1940 in Brooklyn, New York, Fauci attended Cornell University Medical School, graduating in 1966. Three years later, he became a clinical associate in the Laboratory of Clinical Investigation of NIAID. He has worked for the NIH ever since.

From the beginning of his medical career, Fauci has focused on the functioning of the body's immune responses and the impact of infectious diseases on them. By 1971, he had found cures for three diseases affecting body immunity.

When AIDS became recognized in the United States in 1981, Fauci was already deputy clinical director of NIAID. He immediately shifted the focus of the Laboratory of Clinical Investigation to the investigation of AIDS. The lab then made the important discovery of how HIV affects helper T cells.

During his tenure at the NIH, Fauci has worked not only against AIDS but against government indifference to the disease. He has won increasingly larger budgets for research. Fauci and his laboratory teams continue to search for an AIDS vaccine and to develop drug therapies to try to help those people already afflicted.

B CELLS AND IMMUNITY. Antibody-mediated immunity is activated when a B cell encounters its triggering antigen on the surface of a macrophage. B cells can also be activated by chemicals secreted by helper T cells that have already encountered specific antigens. In either case, once a B cell has been activated, it divides numerous times into plasma cells and memory B cells.

The many plasma cells that descend from a B cell have the ability to produce antibodies (also called gamma globulins). These Y-shaped proteins attach themselves to the foreign antigen. Each antibody produced matches only one antigen, much like a key matches a specific lock. Often the fit is precise; other times it is not. Regardless, once the antibody interlocks with the antigen, it either transforms the cell into a harmless substance or marks the entire antibody-plus-foreign cell package for destruction by phagocytes.

Memory B cells perform the same immunity function as memory T cells: to quicken and strengthen the immune response. Long-lived, they remain in the body to face the same antigens should they appear again. Once that exposure takes place, the memory B cells immediately divide and multiply into antibody-secreting plasma cells.

TYPES OF BODY IMMUNITY. The human body is protected by two types of immunity: genetic and acquired.

HOW THE SMALLPOX VIRUS AROSE

The first effective vaccine was developed against smallpox, a fast-spreading disease characterized by high fever and sores on the skin. Prior to the eighteenth century, the disease was common and often fatal. In 1796, while a smallpox epidemic raged in Europe, English physician Edward Jenner (1749–1823) tested a theory. He had observed that people who had been in contact with cows did not develop smallpox. Instead, they developed cowpox, a similar, milder disease of cows that was not a threat to human life. Jenner believed that individuals who had been infected with cowpox had developed an immunity to the more severe human smallpox.

Taking cowpox fluid from the sores of a milkmaid named Sarah Nelmes, Jenner rubbed it into cuts on the arm of eight-year-old James Phipps. A few days later, the boy came down with a mild case of cowpox, but soon recovered. Six weeks later, Jenner gave young Phipps some fluid from a person who had smallpox and he was not affected. His body had developed an immunity against the disease. Jenner called his procedure vaccination, from the Latin vaccinus, meaning "of cows."

In 1980 the World Health Organization declared the eradication of smallpox, the only infectious disease to be completely eliminated.

Genetic or inherited immunity is present in the body from birth. It does not involve an immune response or the production of antibodies. Because of the structure of human DNA (deoxyribonucleic acid; the genetic material determining the makeup of all cells), humans are not subject to certain diseases that dogs and other animals are, and vice-versa. For example, humans cannot contract distemper; however, dogs and cats can. Conversely, humans can suffer from measles; dogs and cats cannot. The genetic makeup of human cells (and of animal and plant cells, also) makes it impossible for certain pathogens to infect and reproduce in those cells.

Acquired immunity is just that: resistance to a pathogen that has been acquired or developed. Involving the production of antibodies, that resistance may evolve actively or passively.

Active immunity is formed when B cells encounter antigens and produce antibodies against them. As an individual grows from birth and develops bacterial and viral infections, antibodies are produced in the body to defend it against subsequent attacks. During a lifetime, an individual's active immunity is continually updated and enlarged. Active immunity may also be gained through artificial means: vaccines. A vaccine is a substance made of weakened or killed bacteria or viruses. When injected (or taken orally) into the body, it stimulates the production of antibodies specific to that particular infectious disease. Too weak to take over the body, the harmful microorganisms are destroyed. In the process, the body has developed memory B cells that can react quickly and effectively when threatened by that particular disease in the future. Developing immunity through deliberate exposure to a disease is called immunization.

Passive immunity differs from active immunity in two ways: the antibodies involved come from an outside source and they provide only short-term resistance. Memory B cells are not produced as a result. An example of passive immunity occurs between a pregnant woman and her fetus. During pregnancy, antibodies produced by the mother cross the placenta (the membrane lining the uterus through which nutrients and oxygen pass from mother to fetus) and enter the fetus's circulation. These antibodies remain for several months after birth, protecting the baby from diseases to which the mother is immune (they remain longer if the mother breast-feeds). They are then naturally removed from the baby's circulation and are not replaced. Antibodies may also be given to individuals who are already fighting infection. Antivenins used to treat poisonous snake or spider bites are examples of such antibody preparations that provide immediate but short-term protection.

AILMENTS: WHAT CAN GO WRONG WITH THE LYMPHATIC SYSTEM

By its very nature, the lymphatic system is involved whenever the body is fighting against foreign pathogens or abnormal body cells. The lymph nodes (especially in the neck) often swell with bacteria and lymphocytes when the body is battling common illnesses such as colds and influenza. However, certain diseases and disorders target the lymphatic system. Some slow down the ability of the system to work; others literally shut it down. The result can be life threatening.

The following are a few of the diseases that can impair the lymphatic system or its parts.

LYMPHATIC SYSTEM DISORDERS

AIDS: Acquired immune deficiency syndrome, a disorder caused by a virus (HIV) that infects helper T cells and weakens immune responses.

Allergy (AL-er-jee): An abnormal immune reaction to an otherwise harmless substance.

Autoimmune disease (au-toe-i-MYOON): Condition in which the body produces antibodies that attack and destroy the body's own tissues.

Graves' disease: Disorder in which an antibody binds to specific cells in the thyroid gland, forcing them to secrete excess thyroid hormone.

HIV: Human immunodeficiency virus, which infects helper T cells and weakens immune responses, leading to the severe AIDS disorder.

Lymphadenitis (lim-fad-e-NIE-tis): Inflammation of lymph nodes.

Lymphangitis (lim-fan-JIE-tis): Inflammation of lymphatic vessels.

Lymphoma (lim-FOE-mah): General term applied to cancers of the lymphatic system, which include Hodgkin's lymphoma and non-Hodgkin's lymphomas.

Multiple sclerosis (skle-ROW-sis): Disorder in which immune cells attack and destroy the insulation covering nerve fibers in the central nervous system, causing muscular weakness and loss of coordination.

Systemic lupus erythematosus (sis-TEM-ick LOU-pus er-i-the-mah-TOE-sis): Also called lupus or SLE, disorder in which antibodies attack the body's own tissues as if they were foreign.

Tonsillitis (tahn-si-LIE-tis): Infection and swelling of the tonsils.

REGIONAL HIV/AIDS STATISTICS: BEGINNING OF 1999

Sub-Saharan Africa: 22,500,000

South and Southeast Asia: 6,700,000

Latin America: 1,400,000

North America: 890,000

East Asia and Pacific: 560,000

Western Europe: 500,000

Caribbean: 330,000

Eastern Europe and Central Asia: 270,000

North Africa and Middle East: 210,000

Australia and New Zealand: 12,000

AIDS/HIV

AIDS (acquired immune deficiency syndrome) has been described as the plague of the twentieth century. Since 1981, when it was first recognized in the United States, the disease has claimed almost 14 million lives worldwide. AIDS is currently the leading cause of death among all men between the ages of twenty-five and forty-four. The World Health Organization (a specialized agency of the United Nations) estimates that at the beginning of the twenty-first century, 40 million people worldwide will be infected with HIV, the virus that causes AIDS. Once infected, individuals may not develop symptoms of the disease for as many as ten years or more.

HIV (human immunodeficiency virus) impairs the body's ability to produce an immune response. Specifically, the virus infects helper T cells. Once inside a helper T cell, HIV can replicate or reproduce within the cell and kill it in ways that are still not completely understood. When the newly formed viruses break out of the dying helper T cell, they continue the cycle by infecting other helper T cells. In response, the body produces more helper T cells, but this only provides the virus with more hosts in which to grow and spread.

Because helper T cells play a central role in directing the body's immune response, their destruction brings about a drop in cell-mediated immunity. The number of antibodies produced in the body declines, leaving it without defenses against a wide range of invaders. Many different types of infections and cancers can develop, taking advantage of the body's weakened immune response. These infections, normally harmless when the body is functioning properly, are known as opportunistic infections.

UNITED STATES HIV/AIDS STATISTICS, 1981–1997

HIV/AIDS WORLWIDE ESTIMATES: BEGINNING OF 1999

People living with HIV/AIDS: 33.4 million

New HIV infections in 1998: 5.8 million

Deaths due to HIV/AIDS in 1998: 2.5 million

Total number of deaths due to HIV/AIDS: 13.9 million

HIV is transmitted between humans in blood, semen, and vaginal secretions. The two main ways to contract the virus are by sharing a needle with a drug user who is HIV-positive or by having unprotected sexual relations with a person who is HIV-positive. (A person who is HIV-positive is already infected with the virus.) It is possible for a pregnant woman who is HIV-positive to transfer the virus to the fetus in her womb. A few individuals have become infected with the virus after receiving a transfusion of contaminated blood.

HIV cannot be transferred through insect bites or stings nor through shaking hands or hugging. No one can contract the virus by sharing telephones or eating utensils, by drinking out of public water fountains, or by swimming in public pools.

There is currently no cure for the disease and no vaccine to prevent its spread. The best defense against AIDS is avoiding sexual contact with infected individuals. Intravenous drug use (injecting drugs into the bloodstream) of any kind should always be avoided. Several antiviral drugs have been developed that slow the progress of the disease in infected individuals. Combinations of these drugs—known informally as cocktails—have proven effective in improving the quality and length of life of AIDS patients, especially those who have been diagnosed in the early stages of the disease.

Medical research findings released in April 1999, however, have shown that the virus can "hide" in memory T cells for up to sixty years. When called upon to fight an infection, such as influenza, the memory T cells could flood an individual's system with HIV.

Allergies

An allergy is an abnormal immune reaction to an otherwise harmless substance. Among the most common of medical disorders, allergies affect an estimated 60 million Americans, or more than one in every five people.

Normally, when a foreign microorganism enters the body, antibodies are produced to bind to the antigens on the foreign particles, and a series of immune reactions take place. When harmless, everyday substances cause the same series of immune reactions, the condition is known as allergy. The offending substance is called an allergen.

Common allergens abound. People may react to airborne particles (plant pollens, animal fur, house dust, cigarette smoke), food (nuts, eggs, fish, milk), drugs (penicillin or other antibiotics), insect bites (bees, wasps, mosquitoes, fleas), or even materials (wool and latex).

Symptoms depend on the specific type of allergic reaction. In the most common type of reaction, antibodies stimulated by the allergen cause certain cells to release histamine into the surrounding interstitial fluid. Histamine causes small blood vessels in the area to expand and become "leaky." Excess fluid and mucus develop, and the common symptoms appear: a runny nose, a scratchy or irritated throat, and red, watery eyes. Allergens that cause a reaction on the skin produce reddened, itchy skin. Those that affect the digestive tract may cause a swelling or tingling in the lips or throat, nausea, cramping, or diarrhea. Most reactions begin within seconds after contact with the allergen and last about half an hour. Some may last from one to several hours after contact.

A large number of prescriptions and over-the-counter drugs can treat the symptoms of allergies. Antihistamines, decongestants, and nasal sprays can all be used to decrease or counteract the effect of histamines. Lotions and creams to reduce skin inflammation caused by allergens are also available.

Avoiding allergens is the best way to limit allergic reactions. This is especially true for food allergies. Learning to recognize and avoid those items that produce an allergic reaction allows most people with allergies to lead normal lives.

Autoimmune diseases

Autoimmune diseases are those in which the body produces antibodies and T cells that attack and damage the body's own normal cells, causing tissue destruction. It is a puzzling phenomenon. The reaction can either take place in a number of tissues at the same time or in a single organ. The following are just a few of the many types of autoimmune diseases.

Graves' disease, also called hyperthyroidism, occurs when an antibody binds to specific cells in the thyroid gland, forcing them to secrete excess thyroid hormone. Symptoms of the condition include weight loss with increased appetite, shortness of breath, tiredness, weak muscles, anxiety, and visible enlargement

of the thyroid gland. Treatments include drugs to stop the hormone production, radioactive iodine to destroy the hormone-producing cells and shrink the enlarged gland, and surgery to remove a part or all of the thyroid.

Multiple sclerosis (MS) is a disease in which immune cells attack and destroy the insulation covering nerve fibers (neurons) in the central nervous system (brain and spinal cord). Once the insulation, called myelin, is destroyed, nerve messages are sent more slowly and less efficiently. As a result, the brain and spinal cord no longer communicate properly with the rest of the body. When this occurs, vision, balance, strength, sensation, coordination, and other bodily functions all suffer. More than 250,000 people in the United States are afflicted with MS. Women are twice as likely to get the disease as men. Drugs have been developed that slow the progress of the disease in many patients, but no cure has yet been found.

Systemic lupus erythematosus (also called lupus or SLE) is a disease in which antibodies begin to attack the body's own tissues and organs as if they

were foreign. The cause of SLE is unknown. It can affect both men and women of all ages, but 90 percent of those afflicted are women. Among the many symptoms of the disease are fevers, weakness, muscle pain, weight loss, skin rashes, joint pain, headaches, vomiting, diarrhea, and inflammation of the lining of the lungs or the lining around the heart. Treatment for SLE depends on how severe the symptoms are. Mild symptoms like inflammation can be treated with aspirin or ibuprofen. Severe symptoms are often treated with stronger drugs, including steroids. Drugs to decrease the body's immune response may also be used for severely ill SLE patients.

Lymphadenitis

Lymphadenitis is the inflammation of lymph nodes. The cause is often an infection of the nodes by bacteria that has entered through a cut or wound in the skin. A virus may also be the cause. The infection may occur in a limited number of nodes in a specific area or in many nodes over a wider area. If the lymph vessels connecting the affected nodes are also inflamed, that condition is known as lymphangitis.

The swollen nodes are often painful to the touch. The skin over the nodes may also be red and warm to the touch. If the accompanying lymph vessels are involved in the infection, they will appear as red streaks from the wound to the lymph nodes. In children, the swollen nodes often appear in the neck because they are close to the ears and throat—locations of frequent bacterial infections in children.

Treatment for lymphadenitis and lymphangitis usually involves medications. Antibiotics, such as penicillin, are often prescribed, and the infection is brought under control in three to four days. If left untreated, the infection may lead to blood poisoning, which is sometimes fatal.

Lymphoma

Lymphoma is a type of cancer in which cells of the lymphatic system (B cells and T cells) become abnormal and begin to grow uncontrollably. Because lymphatic tissue is found throughout the body, lymphomas can occur anywhere. There are many types of lymphomas, but they are generally divided into two main groups: Hodgkin's lymphoma and non-Hodgkin's lymphomas. The exact cause of the cancers in either group is not known.

Hodgkin's lymphoma (or Hodgkin's disease) can occur at any age, although people in early adulthood (ages fifteen to thirty-four) and late adulthood (after age sixty) are most affected. The cancer begins in a lymph node (usually in the neck), causing swelling and possibly pain. After affecting one group of nodes, it progresses on to the next. In advanced cases of the cancer, the spleen, liver, and bone marrow may also be affected. Symptoms

include fatigue, weight loss, night sweats, and itching. As the cancer spreads throughout the body, the immune response becomes less effective. Common infections caused by bacteria and viruses begin to take over.

Hodgkin's lymphoma is one of the most curable forms of cancer. However, as with any form of cancer, early detection and treatment is highly recommended. Once detected in the body, Hodgkin's is usually treated through chemotherapy (using a combination of drugs to kill the cancer cells and shrink any tumors) or radiation therapy (using X rays or other high-energy rays to kill the cancer cells and shrink any tumors) or a combination of both.

Non-Hodgkin's lymphomas encompass over twenty-nine types of lymphomas. Again, the exact cause of these lymphomas is unknown. In general,

males suffer from these cancers more than females. People between the ages of sixty and sixty-nine are at the highest risk of contracting these lymphomas. Non-Hodgkin's lymphomas also tend to strike people suffering from AIDS. Symptoms for non-Hodgkin's lymphomas are similar those for Hodgkin's lymphoma. Along with the swelling of lymph nodes, patients may experience loss of appetite, weight loss, nausea, vomiting, pain in the lower back, headaches, fevers, and night sweats. The liver and spleen may enlarge, as well. Immune responses may be weakened.

Treatment for non-Hodgkin's lymphomas also include chemotherapy and radiation therapy (either by themselves or in combination). In severe cases, bone marrow transplants may take place. Since the "cure" rate for non-Hodgkin's lymphomas is not as good as it is for Hodgkin's lymphoma, early detection and treatment is vital.

Tonsillitis

Tonsillitis is an infection and swelling of the tonsils. The condition is caused by bacteria or viruses that have entered the body through the mouth or sinuses. In addition to swollen and red tonsils, symptoms include a mild or severe sore throat, fever, chills, muscle aches, earaches, and tiredness. Although anyone can be afflicted with tonsillitis, the disease is most common in children between the ages of five and ten.

For mild cases of tonsillitis, treatment usually involves bed rest and drinking extra fluids. The body usually brings the infection under control within a few days. If the case is more severe, penicillin or other antibiotics may be prescribed to combat the infection. If an individual suffers repeatedly from

severe tonsillitis, the tonsils may be removed surgically. That procedure is called a tonsillectomy.

TAKING CARE: KEEPING THE LYMPHATIC SYSTEM HEALTHY

It is important to keep the lymphatic system healthy as it is a vital part of body immunity and overall health. Since the system is closely allied with the cardiovascular system, approaches to keeping that system healthy are recommended for the lymphatic system, also.

The following all play a part in keeping the lymphatic system operating at peak efficiency: proper nutrition, healthy amounts of good-quality drinking water, adequate rest, regular exercise, and stress reduction.

If left unchecked, infection can quickly weaken the body's immune response, leading to serious health problems. It is best to avoid sources of disease, infection, pollution, and other unsanitary substances. Caring for the body by practicing good hygiene will reduce the threat of infection from ever-present bacteria and viruses in the environment. Injuries such as scrapes, cuts, and wounds should be properly cleansed and cared for to prevent infection or the spread of infection. Serious injuries should be treated immediately by qualified medical personnel.

FOR MORE INFORMATION

Books

Aaseng, Nathan. Autoimmune Disease. New York: Franklin Watts, 1995.

Edelson, Edward. The Immune System. New York: Chelsea House, 1990.

Friedlander, Mark P., and Terry M. Phillips, Ph.D. The Immune System: Your Body's Disease-Fighting Army. Minneapolis, MN: Lerner Publications, 1998.

Kendall, Marion. Dying to Live: How Our Bodies Fight Disease. Cambridge, England: Cambridge University Press, 1998.

WWW Sites

Human Body Project: Lymphatic System
http://www.trms.ga.net/~jtucker/students/human/lymphatic
Site provides a brief overview of the lymphatic system: its functions and major organs, its interaction with other body systems, and the various diseases and disorders that can affect it.

Human Lymphatic System Tutorial
http://www.pblsh.com/Healthworks/lymphart.html
An article that provides a nontechnical overview of the lymphatic system, focusing on its parts and what can go wrong.

Lymphatic System
http://www.bae.ncsu.edu/bae/courses/bae495g/1998/slides/lymph/index.htm
Site contains fifty-four slides (links) that provide information or answer questions about the lymphatic system and its various parts.

Lymphoma Resources Page
http://www.alumni.caltech.edu/~mike/lymphoma.html
An extensive and well-organized site presenting information on Hodgkin's disease and non-Hodgkin's lymphoma as they affect both adults and children. Also includes a glossary and a list of books focusing on lymphomas.

National Center for HIV, STD, & TB Prevention
http://www.cdc.gov/nchstp/hiv_aids/dhap.htm
Homepage of the CDC's division of HIV/AIDS prevention. Presents recent information on HIV and AIDS.

Non-Hodgkin's Lymphoma Web Page
http://www.nh-lymphoma.mcmail.com/
A site developed by a sufferer of non-Hodgkin's lymphoma to provide readers with a view of the disease, covering initial symptoms, diagnosis, treatment, and possible cures.

Understanding Lymphoma
http://www.lymphoma.ca/llsdt.htm
A site run by the Lymphoma Research Foundation of Canada that provides an overview of the lymphatic system and the causes, diagnosis, and treatment of lymphomas.

Virtual Anatomy Textbook: The Lymphatic System
http://www.acm.uiuc.edu/sigbio/project/updated-lymphatic/lymph1.html
Site provides a brief but informative look into the body's defense system. Text and illustrations/pictures explain the various parts of the lymphatic system.