A series of gene loci, forming part of the major histocompatibility complex on chromosome 6 in humans, that encode the component polypeptide chains of the MHC class I proteins and MHC class II proteins. These proteins play crucial roles in the immune system, by binding peptide fragments derived from pathogens and presenting these on the surface of host cells for recognition by effector T cells. There are three gene loci encoding the α‎ chain of class I proteins: HLA-A, HLA-B, and HLA-C (the β‎₂-microglobulin is encoded on chromosome 15). Class II α‎ and β‎ chains are encoded by a further three pairs of loci, designated HLA-DP, HLA-DQ, and HLA-DR, with the latter often having an extra β‎ gene. This range of genes means that any individual can produce three different class I proteins and three or four different class II proteins, giving a variety of antigen-binding properties. These MHC glycoproteins also act as antigens themselves and are important in determining the acceptance or rejection by the body of a tissue or organ transplant (see graft). These antigens are one group of the so-called histocompatibility proteins (see histocompatibility). Two individuals with identical HLA types are said to be histocompatible. Successful transplantation requires a minimum number of HLA differences between the donor’s and recipient’s tissues.