histocompatibility

histocompatibility: see transplantation, medical.

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histocompatibility [¦hi·stō·kəm′pad·ə′bil·əd·ē]

(immunology)

The capacity to accept or reject a tissue graft.

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Histocompatibility

A term used to describe the genes that influence acceptance or rejection of grafts. When grafts of tissue are exchanged between genetically dissimilar individuals, profound immunological rejection generally takes place. In contrast, grafts between genetically similar individuals, such as identical twins, are normally tolerated; they are histocompatible. Most known examples of histocompatibility (or H) genes encode polymorphic (that is, tending to differ between individuals) cell-surface proteins.

The major histocompatibility complex (MHC) contains a set of histocompatibility genes, termed major because mismatching at these genes invokes rapid rejection. The main function of MHC genes involves distinguishing self from nonself in the immune system, as part of preventing the spread of infectious disease. The body employs special mechanisms to avoid rejection of the fetus, which is effectively an allograft, that is, a graft from a donor to a genetically dissimilar recipient of the same species; in this case, the mechanisms include a diminution of MHC gene expression.

The MHC contains a spectrum of genes, many of which influence processing and presentation of antigens to the immune system. In mice, the MHC is designated the H-2 complex; in humans, it is referred to as the HLA complex (for human leukocyte A system). Mice and other mammals seem to have a similar arrangement of genes in their MHCs. See Antigen, Cellular immunology, Mendelism, Transplantation biology