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A branch of clinical pathology concerned with the role of the immune defense system in disease. The subject encompasses diseases where a malfunction of the immune system itself is the basic cause, together with diseases where some external agent is the initiating factor but an excessive response by the immune system produces the actual tissue damage. It also extends to the monitoring of the normal immune response in infectious diseases and to the use of immunological techniques in disease diagnosis. See Allergy, Autoimmunity, Hypersensitivity, Immunological deficiency
Many features of the immune system make it prone to shift from protecting the body to damaging it. This complex system not only must distinguish between the body's own cells and a foreign invader but must also recognize and eliminate the body's own cells if they are damaged or infected with a virus. The recognition receptors used to make this fine distinction between “self” and “not self” are not encoded in the genes. Rather, they are assembled following random rearrangement of information carried in small gene segments. During their development, immune system cells are subjected to a selective process, those bearing potentially useful receptors being preserved while those bearing dangerous, self-reactive receptors are eliminated. This process is closely balanced, and some potentially self-reactive cells often persist. See Cellular immunology
There are several approaches to suppressing excessive immune reactivity. Desensitization, or modifying the nature of the response by injecting small amounts of the foreign antigen, is sometimes used to treat allergic states. In contrast, there are few therapies for enhancing immune responses. Bone marrow transplantation is used to restore the immune system in some immunodeficiency diseases. Passive transfer of preformed antibody protects against some infections, and transfusion of immunoglobulin is used to treat immunoglobulin deficiencies. However, vaccination or immunization is one of the most effective of all medical procedures. See Immunosuppression