Every mammalian organism possesses a tightly linked cluster of genes, the major histocompatibility complex (MHC), whose products play roles in intercellular recognition and in discrimination between self and nonself. The MHC participates in the development of both humoral and cell mediated immune responses. While antibodies may react with antigens alone, most T cells recognize antigen only when it is combined with an MHC molecule. The concept that the rejection of foreign tissue is the result of an immune response to cell-surface molecules, originated from the work of Peter Gorer in the mid-1930s. He identified four groups of genes, designated I to IV, that encoded blood-cell antigens. Gorer and George Snell established that antigens encoded by the genes in the group designated II took part in the rejection of transplanted tumors and other tissue. Snell called these genes “histocompatibility genes”; their current designation as histocompatibility-2 (H-2) genes was in reference to Gorer’s group II blood-group antigens. Snell was awarded the Nobel prize in 1980 for this work. Class I and class II MHC molecules are membrane-bound glycoproteins that are closely related in both structure and function. Both class I and class II MHC molecules have been isolated and purified and the three dimensional structures of their extracellular domains have been determined by x-ray crystallography. Both types of membrane glycoproteins function as highly specialized antigen-presenting molecules. The fully formed MHC class I molecule is a heterotrimer consists of α1, α2, α3 and β2-microglobulin chain. MHC class I molecules are made up of two polypeptide chains, α and β2-microglobulin. The “α” chain is around 44 kD and “β2-microglobulin” chain is around 12kD in size. Each α chain is divided into three parts to accommodate extracellular α1, α2, and α3 domains, transmembrane domain and a cytoplasmic tail. The α1 and α2 is around 90 amino acids long and binds to only 8-11 amino acid long peptides (peptide binding cleft). Because MHC class I peptide binding cleft is closed and the larger peptide cannot be accommodated in the designated space. The α1 and α2 contains the polymorphic residues which are responsible for the variation among the MHC I allele and their recognition by a specific T cell. The α3 segment of α chain contains the binding site for CD8+ cells. The α3 segment extends to 25 amino acids residue towards its carboxy terminal covering the lipid bilayer and more 30 amino acids as a cytoplasmic tail. The β2-microglobulin non-covalently interacts with α3 chain. Binding of the peptide in the cleft between α1 and α2 strengthens the interaction between α and β2- microglobulin chain. MHC class II molecules are also made up of two polypeptide chains, α and β. The “α” chain is around 33 kD and “β” chain is around 28kD in size. Like class I chains, class II MHC molecules are membrane-bound glycoproteins that contain external domains, a transmembrane segment.