human leukocyte antigen (HLA), antigens formed from genes on chromosome 6. The loci are identified by numbers and letters, such as HLA-B27. Antigens are divided into three classes. Class I antigens (HLA-A, -B, and -C) occur on the surface of all nucleated cells and platelets and present endogenous antigen peptides to CD8 T cells (in other mammals: major histocompatibility complex [MHC] class I molecules). If donor and recipient HLA complexes do not match, the nonself antigens are recognized as foreign and destroyed by CD8 T cells. Class II antigens occur only on antigen-presenting cells and present exogenous antigens (in nonhumans MHC class II molecules). Class III antigens are nonhistocompatibility antigens, such as some complement components, that map in the HLA complex. Also called major histocompatibility complex. See also HLA complex.
