major histocompatability complex
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major histocompatability complex (MHC) a cluster of tightly linked genes on chromosome 6 in humans. These genes code for protein molecules (MHC molecules or MHC antigens) that are attached to the surface of body cells and used by the body's immune system to recognize its own or foreign material. Class 1 MHC antigens are located on the surface of virtually all human cells, but Class II MHC antigens are restricted to MACROPHAGES and the antibody-producing B-LYMPHOCYTES. See also HLA SYSTEM.