Major Histocompatibility Complex (MHC) 
(En español)
A group of molecules found on the surface of almost every cell in the body. Major histocompatibility complex (MHC) molecules present protein fragments (peptides) to immune cells. If the cells recognize the peptides as foreign, the body mounts an immune response. In humans, MHC molecules are called human leukocyte antigens (HLAs).