major histocompatibility complex

n. (Abbr. MHC)
A group of genes that code for cell-surface histocompatibility antigens and are the principal determinants of tissue type and transplant compatibility.

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n.

(Abbr. MHC) A chromosomal segment that codes for cell-surface histocompatibility antigens and is the principal determinant of tissue type and transplant compatibility. Also called HLA complex.

abbr.: MHC; a complex of genetic loci occurring in higher vertebrates that encodes a family of cellular antigens, known in mice as histocompatibility-2 antigens (abbr.: H-2 antigens; see histocompatibility antigen) and in humans as human leukocyte-associated antigens (abbr.: HLA antigens; see HLA histocompatibility system). The MHC antigens are cell-surface glycoproteins and may be divided into two classes, designated I and II. In transplantation reactions, cytotoxic T lymphocytes respond mainly against foreign MHC glycoproteins of class I together with antigen, while the response against class II and antigen is mainly by helper T lymphocytes.

Class I MHC antigens (or transplantation antigens) are on the surface of most nucleated somatic cells and enable the immune system to distinguish self from foreign cells. They are encoded by three separate loci (H-2K, H-2D, and H-2L in mice; HLA-A, HLA-B, and HLA-C in humans). The antigenic activity resides in a transmembrane polypeptide chain, the α subunit, of ≈45 kDa (roughly 345 amino acids), which has a short hydrophilic C-terminal segment inside the cell and a large glycosylated N-terminal segment. The latter is folded into three separate domains (two of which contain an intrachain disulfide bond) and exposed to the cell exterior, where it is noncovalently linked with the β subunit (of 11.5 kDa and also called β₂ microglobulin), which is homologous with a single Ig-like domain (see immunoglobulin fold). In humans, these proteins are also referred to as HLA class I.

Class II MHC antigens are found on the surface of most B lymphocytes, some T lymphocytes, and some macrophages and macrophage-like cells. They are encoded by murine I-region (abbr.: Ir) genes and human HLA-D genes, and contain a ≈33 kDa α chain and a 28 kDa β chain; the C-terminal regions of both subunits have transmembrane and intracellular domains. Celiac disease is due to an allelic variant in one class-II gene.

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Major histocompatibility complex.

n

The genetic region that contains the loci of genes that determine the structure of the serologically defined (SD) and lymphocyte-defined (LD) transplantation antigens, genes that control the structure of the immune response-associated (Ia) antigens, and the immune response (Ir) genes that control the ability of an animal to respond immunologically to antigenic stimuli.