Genetically encoded cell-surface alloantigens that can cause the rejection of grafted tissues, cells, and tumors bearing them. These cell-membrane glycoproteins are grouped into two classes. Class I molecules are found on the surfaces of all mammalian cells (except trophoblasts and spermatozoa). T lymphocytes (q.v.) of the CD8+ subgroup recognize antigenic determinants of foreign class I histocompatibility molecules. Class II histocompatibility molecules are abundant on the surfaces of B lymphocytes (q.v.). T lymphocytes of the CD4+ subgroup recognize antigenic determinants of foreign class II histocompatibility molecules. These T cells subsequently divide and secrete lymphokines (q.v.), which are important for B cell growth and differentiation. Class I histocompatibility molecules are heterodimers made up of heavy (alpha) and light (beta) polypeptide chains. The class I chains are encoded by genes residing in the right portion of the HLA complex (q.v.). The alpha chain contains regions showing sequence diversity, whereas the beta chain has an invariant amino acid composition. The class II histocompatibility molecules are also dimers composed of heavy alpha and light beta chains. These are encoded by genes in the left portion of the HLA complex. Most of the sequence diversity of class II histocompatibility molecules is localized within a segment of the beta chain. Class II histocompatibility dimers are associated with a third polypeptide chain that exhibits no polymorphism. See Chronology, 1937, Gorer; 1987, Wiley et al.; major histocompatibility complex.
Subjects: Genetics and Genomics.