HLA complex, antigens formed from genes on chromosome 6. HLA genes code for proteins that enable the immune system to differentiate tissues or proteins between “self” and “nonself.” These 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 are important in tissue transplantation. If donor and recipient HLA antigens do not match, the nonself antigens are recognized and destroyed by killer T cells. Class II antigens occur only on immunocompetent cells and normally recognize foreign proteins. Class III antigens are nonhistocompatibility antigens, such as some complement components, that map in the HLA complex. Also called major histocompatibility complex, histocompatibility complex.
