MHC molecules as peptide receptors

The central unit for regulation of the specific immune system is a trimolecular complex made up of the T cell antigen receptor, the MHC molecule, and the MHC ligand. The third component is a peptide derived as a degradation product from a protein. During recent years there has been some progress in understanding the interaction between MHC molecules and their peptide ligands: MHC molecules are peptide receptors of peculiar specificity, being able to accomodate millions of different peptides provided they share some common features.