Needles in haystacks: identifying specific peptide antigens for T cells.

The antigen receptors of T cells are engaged by unique peptides displayed by MHC molecules on the antigen-presenting cell surface. These rare peptides can now be distinguished among the thousands normally bound to MHC molecules. Knowledge of the antigenic peptides and their donor proteins is suggesting potential targets for immunotherapy and providing insights into the cellular mechanisms that generate the peptide-MHC complexes.