immunology, branch of medicine that studies the response of organisms to foreign substances, e.g., viruses, bacteria, and bacterial toxins (see immunity). Immunologists study the tissues and organs of the immune system (bone marrow, spleen, tonsils, thymus, lymphatic system), its specialized cells (e.g., B and T lymphocytes and antibodies), and the influence of genetic, nutritional, and other factors on the immune system. They also study disease-causing organisms to determine how they injure the host and help to develop vaccines (see vaccination).

In addition to studying the normal workings of the immune system, immunologists study unwanted immune responses such as allergies, essentially immunological responses of the body to substances or organisms that, as a rule, do not affect most people, and autoimmune diseases (e.g., rheumatoid arthritis and lupus erythematosus) which occur when the body reacts immunologically to some of its own constituents.

Immunologists have developed a large number of procedures have been developed to detect and measure quantities of immunologically active substances such as circulating antibodies and immune globulins. Immune globulins that can be given intravenously (IVIGs) have been found to be more effective against antibody deficiencies and certain autoimmune diseases than their older intramuscular counterparts; their use in a wide spectrum of bacterial and viral infections is under study. Current research in immunology is also aimed at understanding the role of T lymphocytes (see immunity), which play a major part in the body's defenses against infections and neoplasms. AIDS, for example, is the disease that results when the HIV virus destroys certain of these T cells.

Science dealing with the body's defenses against disease-causing microorganisms and disorders of those defenses. Starting with Edward Jenner's use of a vaccine against smallpox in 1796, immunology has arrived at a comprehensive and sophisticated understanding of the role of microorganisms in disease and of the formation, mobilization, action, and interaction of antibodies and antigen-reactive cells. It covers treatment of allergies, immunosuppression after organ transplants to prevent rejection, and study of autoimmune diseases and immunodeficiencies. AIDS has stimulated intensive research in the last of these.

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