A large gene cluster that encodes various components of the immune system, including the histocompatibility proteins and components of the complement system. In humans the MHC contains over 200 genes, is located on chromosome 6, and includes the HLA system. Other vertebrate species have similar MHC regions. Certain MHC genes can have many variant alleles; this produces an enormous diversity of proteins in a population, each individual possessing a unique set. See mhc class i protein; mhc class ii protein.