MAJOR HISTOCOMPATIBILITY COMPLEX

The major histocompatibility complex (MHC) is a large genomic region or gene family found in most vertebrates. It is the most gene-dense region of the mammalian genome and plays an important role in the immune system, autoimmunity, and reproductive success. The proteins encoded by the MHC are expressed on the surface of cells in all jawed vertebrates, and display both self antigens (peptide fragments from the cell itself) and nonself antigens (e.g. fragments of invading microorganisms) to a type of white blood cell called a T cell that has the capacity to kill or co-ordinate the killing of pathogens, infected or malfunctioning cells.
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Major histocompatibility complex n. Abbr. MHC A chromosomal segment that codes for cell-surface histocompatibility antigens and is the principal [more]Major histocompatibility complex - Community and Resources

(MHC): A genetic complex of vertebrates consisting of around 100 genes including the extremely polymorphic cell surface molecules called HLA in humans and H-2 in mice. These molecules provide an immunological marker for selfness and a genetic self-identity to the individual. This information is used in mate choice, union of gametes, maintenance of pregnancy, and immune response against nonself (including a transplanted graft). These molecules are the most polymorphic ones in vertebrates. The polymorphism arises from point mutations not at an unusually higher rate than other genes, and mainly from interallelic gene conversion events. The polymorphism is maintained through pathogen and non-pathogen driven mechanisms via heterozygous advantage (overdominant selection) and negative frequency dependent selection. A 3.6 Mb long human MHC haplotype and the 92 kb chicken MHC have been totally sequenced (see Nature 1999(Oct 28);401:921-925).