[Human histocompatibility HLA system. Considerations in the light of current concepts. II. Immunogenetic function]

Investigation of self-nonself recognition in the immune system has focused on classical class I and class II MHC antigens. Their extensive polymorphism and high level expression rendered them amenable to experimentation with the traditional immunologic tools, serology and transplantation. Much has been learned about their functions as a peptide--presenting receptors important for immune responses against pathogens. The central unit for regulation of the specific immune system is a trimolecular complex made up of the T cell antigen receptor (TCR), 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 accommodate millions of different peptides provided they share some common features. In this review is to discuss the relationship between MHC molecules and their natural peptide ligands with special emphasis on MHC class I molecules, because more is known about these than class II molecules.