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immune system

A sophisticated and diverse system found in vertebrates whose primary task is to defend them against foreign organisms, usually viruses and bacteria, by recognizing the difference betweenâ€˜selfâ€™ andâ€˜non-selfâ€™. The differentiated cells that form the immune system are the leucocytes and include macrophages, which engulf and digest foreign matter by phagocytosis, and lymphocytes, which aid the recognition of invading organisms. Some lymphocytes (in particular the B cells, some of which mature into plasma cells) produce antibodies which selectively bind to a particular arrangement of atoms on the surface of foreign organisms called an antigen. The reaction between an antibody and an antigen is termed an immune response (See T cell). Immune deficiency viruses, including HIV, attack the immune system and render the animal susceptible to secondary infections to produce the symptoms of AIDS. Problems frequently arise during transplant operations in which tissue from a donor implanted into the recipient patient is rejected as a result of the immune system\'s ability to recognize foreign cells. In such cases the immune response is moderated by the use of immuno-suppressive drugs. A number of components of the immune system are encoded by the major histocompatibility complex (MHC). This is a large cluster of genes on chromosome 6 in humans. There are many variations of the genes found in this cluster, and their characterization is the basis of tissue typing for transplant operations.

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