ABSTRACT

The key initial step for most immune responses is T-cell recognition of an antigen not in its intact form but rather as peptide fragments derived from the foreign protein and bound to MHC molecules. This antigen presentation step is necessary for triggering of effector T cells, for provision of “help” for B-cell activation and differentiation into immunoglobulin (antibody)-producing cells, and for activation of macrophages. Multiple cell-surface molecules interact during this antigen presen­ tation step but the trimolecular complex - MHC/antigenic peptide/TCR - is the focal point for initiation and propagation of most immune responses to protein antigens.