The major histocompatibility complex (MHC) is a region of highly polymorphic
genes whose products are expressed on the surfaces of a variety of cells.
• Ag-specific T cells do not recognize antigens in free or soluble form but recognize
peptide form of protein antigens that are non-covalently bound to MHC gene products
• There are two major types of MHC gene products: MHC class I and class II