thumb|Right|300px|Protein images comparing the MHC I (1hsa) and MHC II (1dlh) molecules. (discussion/molecule_of_the_month/pdb62_1.html" target="_blank">more details...) The major histocompatibility complex (MHC) is a large genomic region or gene family found in most vertebrates. It is the most gene-dense region of the mammalian genome and plays an important role in the immune system, autoimmunity, and reproductive success. The proteins encoded by the MHC are expressed on the surface of _cells in all jawed vertebrates, and display both self antigens (peptide fragments from the cell itself) and nonself antigens (e.g. fragments of invading microorganisms) to a type of white blood cell called a T cell that has the capacity to kill or co-ordinate the killing of pathogens, infected or malfunctioning cells.