The
Major Histocompatibility Complex (MHC) is a family of genes that encode the creation of certain
cell surface
glycoproteins important in the immune response.
(Somewhat confusingly, the abbreviation MHC also refers to the gene product, the MHC molecule. To disambiguate the usage, some of the biomedical literature uses Mhc to refer specifically to the molecule, and reserves MHC for the region of the genome that encodes for this molecule, however this convention is not consistently adhered to).
MHC molecules (the glycoproteins encoded by the family of genes of the major histocompatibility complex) receive antigens from inside the cells they are part of and display them on the cell's exterior surface for recognition by T cells.
- Class I MHC molecules are found on almost every nucleated cell of the body. Class I molecules are heterodimers, consisting of a single transmembrane polypeptide chain (the α-chain) and a β2 microglobulin (which is encoded elsewhere, not in MHC cluster of genes). The α chain has two polymorphic domains: α1, α2 which present peptides derived from cytosolic proteins to the immune system. The peptides consist of 8-10 amino acid residues.
- Class II MHC molecules are found only on a few specialized cell types, including macrophages, activated T cells, and B cells. Class II molecules are also heterodimers consisting of two polypeptide chains (an α and β chain). Class II molecules also present peptides, but unlike class I, they are derived from endocytosed plasma membrane and extracellular proteins. The peptides are generally between 15-24 amino-acid residues.