histocompatibility

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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 also Antigen; Cellular immunology; Mendelism; Transplantation biology.

The quality of a cellular or tissue graft enabling it to be accepted and functional when transplanted to another animal.

• h. antigen — genetically determined antigens present on the cell membranes of nucleated cells of most tissues, which incite rejection when tissues are grafted to a different individual and thus determine the compatibility of tissues in transplantation. Major histocompatibility antigens exert the strongest effect on transplanted tissues, while the many minor histocompatibility antigens, such as blood group substances and the secretory alloantigen system, have a weaker, though often highly significant role in graft rejection.
• h. genes — genes that code for the histocompatibility antigens.
• major h. complex (MHC) — a cluster of loci on one autosomal chromosome containing the genes that code for the major histocompatibility antigens. The best studied MHC is the H₂ complex of the mouse followed by the HLA (human leukocyte antigen) of humans. In other species it is further specified: the dog leukocyte antigen (DLA), the bovine leukocyte antigen (BOLA), the swine leukocyte antigen (SLA), etc.
— MHC class I is a polymorphic set of transmembrane heterodimeric proteins consisting of an α chain, encoded within the MHC gene complex, associated noncovalently with β₂-microglobulin expressed on nearly all nucleated cells and which form a trimeric complex with typically 9-mer peptides that are then transported and presented on the surface of the cells and serve as targets for cytotoxic T lymphocytes (CTL or Tc), CD8 cells. A major defense mechanism against viral infections. — MHC class II is a polymorphic set of transmembrane heterodimeric proteins that consist of noncovalently associated α and β proteins, both encoded within the MHC gene complex. They form a trimeric complex with antigenic peptides, which may be up to 35-mers, in antigen presenting cells which are then transported to the surface of the cell for presentation to T helper lymphocytes. The basis for all T lymphocyte responses.

The compatibility of the antigens of donor and recipient of transplanted tissue.

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Histocompatibility is the property of having the same, or mostly the same, alleles of a set of genes called the major histocompatibility complex. These genes are expressed in most tissues as antigens, to which the immune system makes antibodies. The immune system at first makes antibodies to all sorts of antigens, including those it has never been exposed to, but stops making them to antigens present in the body. If the body is exposed to foreign antigens, as by getting a tissue graft, it attacks the foreign material unless it is histocompatible.

A similar system exists in many plants; its purpose is not to reject grafts but to prevent inbreeding. Pollen from a plant sharing a histocompatibility gene with the female either fails to grow or dies soon after germinating.

See also

• Transplant rejection
• Tissue typing
• Major histocompatibility complex (MHC)
• Clonal selection theory

External links

• MeSH Histocompatibility
• Histocompatibility at eMedicine Dictionary

This entry is from Wikipedia, the leading user-contributed encyclopedia. It may not have been reviewed by professional editors (see full disclaimer)

Common misspelling(s) of histocompatibility

• histocompatability