MAJOR HISTOCOMPATIBILITY COMPLEX (MHC)
Human histocompatibility antigens are also known as human leucocyte antigens (HLA), a term that are synonymous with the MHC complex. These antigens are cell surface glycoproteins classified as type I and type II. They can produce genetic polymorphism with multiple alleles at each site, thus permitting a great deal of genetic variability between given individuals. This extensive polymorphism is important when viewed in the context of an immune system that needs to cope with an ever-increasing range of pathogens. These pathogens in turn are extremely adept at evading the immune system. Thus, the battle between invading microbe and immune recognition is constant and ever changing. Recognition of antigen by T cells is MHC restricted. Therefore, any given individual is only able to recognize antigen as part of a complex of antigenic peptide and self.