major histocompatibility complex, a molecule that presents antigen peptides to T cells on the surface of cells. Two major classes have been described: major histocompatibility complex (MHC) class I and class II. MHC class I is present on all cells and presents endogenous peptides to CD8 T cells. MHC class II is present only on antigen-presenting cells and presents exogenous peptides to CD4 T cells. MHC, or human leukocyte antigen (HLA) as it is called in humans, also determines the compatibility of tissues or organs transplanted from one individual to another of the same species or from one species to another. See also HLA complex, human leukocyte antigen, antigen processing.
