The ternary complex: T cell receptor, MHC protein, and immunogenic peptide.

The identification and sequencing of the antigen receptor of T cells coupled with the demonstration that MHC proteins specifically bind immunogenic peptides, and the solution of the crystal structure of HLA A2 and Aw68 collectively have led to a working model of how T cells recognize protein antigens. In contrast with many other known receptor-ligand interactions, this unique recognition mechanism has evolved to allow receptors on two separate cells to contact a common peptide ligand. To accomplish this, MHC proteins and the T cell receptor both differ from previously defined biological receptors in many respects. The MHC class I and II molecules are membrane glycoproteins that have evolved the remarkable capacity to bind and display on the surface of cells an extremely large number of structurally diverse peptides, while the antigen specific receptors of T cells are positively selected to specifically interact with the MHC protein alleles of the individual and only some of the repertoire of self peptides.