Antiserum and antitoxin

Both antisera and antitoxins are means of proactively combating infections. The introduction of compounds to which the immune system responds is an attempt to build up protection against microorganisms or their toxins before the microbes actually invade the body.

The use of antiserum and antitoxin preparations is now a standard avenue of infection control . The beginnings of the strategies dates to the time of Edward Jenner in the late eighteenth century. Then, Jenner used an inoculum of cowpox material to elicit protection against the smallpox virus.

Jenner's strategy of using a live organism to elicit an antibody response led to a "third-party" strategy, whereby serum is obtained from an animal that has been exposed to an antigen or to the microorganism that contains the antigen. This so-called antiserum is injected into the human to introduce the protective antibodies directly, rather than having them manufactured by the person's own immune system.

The same strategy produces antitoxin. In this case, the material injected into the animal would consist of active toxin, but in very low quantities. The intent of the latter is to stimulate antibody production against a toxin that has not been changed by the procedures used to inactivate toxin activity.

The use of antitoxin has been largely supplanted by the injection of a crippled form of the toxin of interest (also known as a toxoid) or a particularly vital fragment of the toxin that is needed for toxic activity. The risk of the use of a toxoid or a fragment of toxin is that the antibody that is produced is sufficiently different from that produced against the real target so as to be ineffective in a person.

Since the time of Jenner, a myriad of antisera and antitoxins have been produced against bacterial, viral and protozoan diseases. The results of their use can be dramatic. For example, even in the 1930s, the form of influenza caused by the bacterium Hemophilus influenzae was almost always lethal to infants and children. Then, Elizabeth Hattie, a pediatrician and microbiologist, introduced an anti-influenzal antiserum produced in rabbits. The use of this antiserum reduced Hemophilus influenzae influenza-related mortality to less than twenty per cent.

Antiserum can contain just one type of antibody, which is targeted at a single antigen. This is known as monovalent antiserum. Or, the antiserum can contain multiple antibodies, which are directed at different antibody targets. This is known as polyvalent antiserum.

The indirect protective effect of antiserum and antitoxin is passive immunity . That is, a protective response is produced in someone who has not been immunized by direct exposure to the organism. Passive immunity provides immediate but temporary protection.

Antiserum and antitoxin are obtained from the blood of the test animal. The blood is obtained at a pre-determined time following the injection of the antigen, microorganism, or toxoid. The antiserum constitutes part of the plasma, the clear component of the blood that is obtained when the heavier blood cells are separated by spinning the blood in a machine called a centrifuge.

Examples of antisera are those against tetanus and rabies . Typically, these antisera are administered if someone has been exposed to an environment or, in the case of rabies, an animal, which makes the threat of acquiring the disease real. The antisera can boost the chances of successfully combating the infectious organism. After the threat of disease is gone, the protective effect is no longer required.

The advent of antibiotics has largely replaced some types of antiserum. This has been a positive development, for antiserum can cause allergic reactions that in some people are fatal. The allergic nature of antiserum, which is also known as serum shock, arises from the nature of its origin. Because it is derived from an animal, there may be components of the animal present in the antiserum. When introduced into a human, the animal proteins are themselves foreign, and so will produce an immune response. For this reason antiserum is used cautiously today, as in the above examples. The risk of the use of antiserum or antitoxin is more than compensated for by the risk of acquiring a life-threatening malady if treatment is not undertaken.

Serum sickness is a hypersensitive immune reaction to a contaminating animal protein in the antiserum. The antibodies that are produced bind to the antigen to make larger particles called immune complexes. The complexes can become deposited in various tissues, causing a variety of symptoms. The symptoms typically do not appear for a few weeks after the antiserum or antitoxin has been administered.

With the development of sophisticated techniques to examine the genetic material of microorganisms and identify genes that are responsible for the aspects of disease, the use of antiserum and antitoxin may enter a new phase of use. For example, the genetic sequences that are responsible for the protein toxins of the anthrax bacterium are now known. From these sequences the proteins they encode can be manufactured in pure quantities. These pure proteins can then form the basis of an antitoxin. The antibodies produced in animals can be obtained in very pure form as well, free of contaminating animal proteins. These antibodies will block the binding of the toxin to host tissue, which blocks the toxic effect. In this and other cases, such as an antitoxin being developed to Escherichia coli O157:H7, the use of antitoxin is superior strategy to the use of antibiotics. Antibiotics are capable of killing the anthrax bacterium. They have no effect, however, on action of the toxin that is released by the bacteria .

See also Anti-adhesion methods; Antiviral drugs; E. coli 0157:H7 infection; Escharichia coli ; Immune stimulation, as a vaccine; Immunization