histocompatibility antigen
A set of plasmalemmal glycoproteins that are crucial for T-cell recognition of antigens. Particularly the HLA system in humans and the H2 system in mice. There are two classes of histocompatibility antigens: (1) Class I; histocompatibility antigens composed of two glycosylated subunits, a heavy chain of 44 kD and b 2-microglobulin (12 kD). The heavy chain may be coded by K, D or L genes of mouse H2 and A, B or C genes of human HLA complex. Class I antigens are important in T cell killing and are recognized in conjunction with the foreign cell surface antigens (MHC restriction).
(2) Class II antigens; heterodimeric histocompatibility antigens composed of a (32 kD) and b (28 kD) chains. Found mostly on B-lymphocytes, macrophages and accessory cells. The response of T-helper cells requires that the foreign antigen is presented in conjunction with the appropriate Class II antigens. (Murine H2 Ia antigens and human HLA-DR antigens are Class II).

Dictionary of molecular biology. 2004.

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