The human major histocompatibility complex (MHC) and its genes
In the human, MHC loci are located within a 4 million nucleotide base-pair (bp) stretch on the short arm of the chromosome 6 in the region 6p21.3 and it is referred to as the HLA system, because their protein products were first identified on the surface of human leucocytes. There are two main classes of proper MHC/HLA genes, working as antigen-presenting molecules, class I and class II, which are also the main determinants of histocompatibility. However, a series of genes with different roles in the immune system, including cytokines, heat-shock proteins, enzymes for steroid metabolism and proteins involved in the complement cascade and antigen-processing are located within the MHC loci in between these two classes. They are referred to as class III MHC molecules by some authors, but others believe this is a misnomer as they are genetically different and have different functions from proper MHC molecules.