The specific resistance to disease is called the third line or teritary line of defense. This line of defense is commonly called your immune system. The human immune system begins when the fetus is about 2 months after conception. Two types of cells develop: cells that become red blood cells (erythrocytes) and cells that become lymphocytes. These third line defense cells (lymphocytes) total about 2.2 pounds in a 150 pound human. The two major types of lymphocytes are called T-cells and B-cells. You probably have heard about T-cells---these cells are greatly reduced by the HIV (AIDS virus) and AIDS patients often develop cancers and other opportunistic infections because their third line of body defense is greatly compromised or weakened. T-cells mature in a flat organ in your chest called the thymus; and T-cells defend by destroying abnormal cells. The B-cells probably mature in bone marrow, liver, or spleen and these lymphcytes are thought to concentrate in body fluids and destroy toxins and other potentially harmful substances.

In addition to the lymphocyte attack on pathogens, the immune system also produces blood proteins called antibodies or immunoglobulins (Ig). These blood proteins are produced in response to antigens (foreign materials including microbes) that enter the body. These antibodies remain in the blood and lymph providing defense against future attacks by the same microbe. Antibodies have been studied extensively since the 1950s; and we now know that antibodies are a complex protein molecule. The five most common groups of antibodies are abbreviated IgM, IgG, IgA, IgE, and IgD. About 80% of the total antibody count is IgG and appears about 24-28 hours after contact with the foreing material. IgG also crosses the placenta and provides infant immunity for about 6 months after birth. The antibody IgA is commonly found in tears, saliva, and colostrum. IgE is the antibody that is related to allergic reactions. The variety of antibodies have a varietyof defensive actions. Certain antibodies neutralize virus capsids and toxins and are called antitoxins. Still other antibodies attack the surface of bacteria causing the microbes to clump (agglutinins) and become more sensitive to phagocytosis. Other antibodies are called precipitins because these react with dissolved antigens to form solid precipitates that can be more easily phagocytized. Another type of antibody is called an opsonin and attaches to baceteria like Streptococcus thus making the bacteria unable to resist phagocytosis.

Immunity means "safe" or "free from." This does not mean safe from all diseases, but rather some specific microbe or disease. Immunity is considered natural or artificial.

Natural immunity results passively from congenital immunity. Maternal antibodies cross the placenta and remain in the child for 3-6 months and disappear as the child's immune system starts to perform. Maternal antibodies (IgA) are also present in colostrum (first milk) which provide protection against intestinal pathogens. Some scientists predict that future mothers may be immunized against microbes that cause disease of newborns; then the mother's milk would immunize the infant. Other antibodies known to cross the placenta increase resistance against Bordetella (whooping cough), Staphylococcus, and viral respiratory diseases. The second method of natural immunity results from having the disease.

Articifial immunity results from the active response to a vaccination and the passive response to the injection of antibodies. The passive injection of antibody-rich serum is used for viral diseases like hepatitis-B, encephalitis, and chickenpox as well as bacterial diseases like botulism, diptheria, and tetanus. This passive immunity provides immediate protection but is very temporary (days) in its protection. Numerous vaccines are currently in use to artificially produce temporary or long term immunity. Anti-bacterial vaccines are available for Vibrio (cholera), Haemophilus (meningitis), Bordetella (whooping cough or pertussis), Yersinia (plague), Streptococcus (pneumonia), Rickettsia (Rocky Mountain spotted fever), and Mycobacterium (tuberculosis). Antiviral vaccines are available for hepatitis-B, influenza, measles, mumps, rubella, polio, rabies, chickenpox, and yellow fever. Vaccines are made from dead microbes incapable of multiplying in the body or weakened microbes that multiply at a very slow rate. To avoid multiple injections, some combination vaccines like DPT (diptheria, pertussis, and tetanus) and MMR (measles, mumps, and rubella) are used. The pertussis or whooping cough vaccine provides life-long immunity, but health workers in areas known to have typhoid fever, plague, cholera, Q-fever, and Rocky Mountain spotted fever should receive an updated vaccination. In the future, the use of synthetic vaccines will reduce the use of human blood in the production of vaccines. This will relieve the fear of contracting microbes such as HIV. One example of a recently manufactured synthetic vaccine which does not contain any human blood fragments if Recombivax for hepatitis-B. Oral consumption and nasal sprays may also have increased use for vaccinations in the future.

Even though no vaccination is completely safe, the risk of the dangers from vaccinations is much less than the risks of the dangers from having the disease. Why don't all people have their vaccinations? For instance in 1990, it was estimated that only 70% of infants (less than 2 years of age) had been immunized against measles. It is not necessary that 100% of the population be immunized in order to control a disease; but recent overcrowding and extensive world travel emphasize the reasons for better vaccination procedures. During recent decades the public has become less fearful of contracting diseases like pertussis, polio, and tetanus. Some microbiologists believe that 70% immunization is a minimum to prevent the outbreak and spread of most diseases such as polio. Herd immunity is the resistance of a group to the spread of an infectious microbe resulting from immunity of a high proportion of the members of the group. For a highly infectious disease like influenza, about 90-95% of the population must be immune in order to have herd immunity. Outbreaks of TB and diptheria in crowded population (high density) areas can be prevented only if more that 70% of the individuals are immunized.

Reasons that may be responsible for the failure to have vaccinations include fear of the rare death, fear of side effects (l in every 3 million polio vaccinations results in paralysis; 1 in every 300,000 getting the pertussis vaccination may have brain damage), fear of getting the disease, allergic reactions, poor vaccine quality, cost, convenience, ignorance, and religious beliefs.s The 1990s were thought to be the Golden Age of Vaccines, but much remains to be done.

During the investigations related to immunity, many other related topics were better understood. Hypersensitivity and auto-immune diseases are related to the ability of the individual to recognize antigens. The immune response includes numerous conditions ranging from slight allergies to a very dangerous immune deficiency called SCID (Severe Combined Immunodeficiency Disease). The longest survivor of SCID was David who lived in a sterile plastic area for 12 years, received a bone marrow transplant, and died in 1984 from a virus assumed to have been related to the transplant.

Hypersensitivity (allergy) was first reported in the early 1900s. Immediate and/or delayed response to antigens that result in constriction of breathing tubes, mucous production, edema, itching, rash, and other symptoms is described as hypersensitivity. During the destruction of antigens (foreign materials) by antibodies, histamines are released which produce conditions like hay fever, asthma, and exzema. The allergic reaction may be life-threatening so it is important that anti-histamines are available to reduce symptoms. It is better to avoid the allergen or desensitize the individual prior to experiencing the allergic response. Injections of small but increasing amounts of the allergen or injection of antibodies or antibody fragments to reduce the involvement of IgE (antibody known to be involved in many allergic reactions) seems to be the preferered action to reduce hypersensitivity. Numerous skin and blood (RAST) tests are available to identify and predict the possibility of allergic reactions.

In some individuals, self-antigens are attacked by the individual's own immune system. These, often slow and progressive, disorders are called auto-immune diseases. Certain auto-immune diseases are very organ specific--examples include Hashimotos Disease (thyroid), juvenile diabetes (pancreas), Addison's Disease (adrenal glands), good pasture's syndrome (kidney), multiple sclerosis and Hodgkin's Disease (brain), and Crohn's Disease (intestine). Other auto-immune diseases like rheumatoid arthritis, some cases of male iknfertility, systemic lupus, Guillain-Barre, and pernicious anemia are less organ specific. Auto-immune diseases are often managed through diet and stress reduction. Immunosuppressive therapy is also used to manage these diseases; and individuals must be aware of the increased chance of opportunistic infections. Many auto-immune diseases appear to be related to genetics and the inheritance of certain antigens and hormone levels, as well as the presence of pathogenic bacteria and viruses.