Major histocompatibility complex

The cell-mediated adaptive immune response is regulated by the major histocompatibility complex (MHC). A major role of the MHC is to bind small peptides and to present them to the cell surface where the antigen can be recognized by T cell receptors. MHC molecules display both ‘self’ peptides derived from their own proteins, and foreign peptides derived from invading pathogens. The immune system is constantly monitoring the surfaces of cells, and the MHC-presented peptides help immune cells to discriminate between normal antigens on the surface of all cells, and those that are foreign and potentially dangerous. Defects in certain MHC genes lead to autoimmune disorders in which the body fails to recognize self-antigens, such as occurs in diseases like multiple sclerosis, inflammatory bowel disease, and in some forms of arthritis and diabetes. There are two major classes of MHC molecules. MHC class I molecules (MHC I) consist of one membrane-spanning a chain (heavy chain) produced by MHC genes, and one b chain (light chain or b2-microglobulin) produced by the b2-microglobulin gene. They are present on most cells and bind CD8 adhesion molecules on cytotoxic T cells. MHC class II molecules (MHC II) consist of two membrane-spanning chains, a and b, of similar size and both produced by MHC genes. They are present on antigen-presenting cells and bind CD4 adhesion molecules on helper T cells.