The MHC ability to show a vast range of antigenic peptides for T cell recognition needs a compromise between broad specificity and high affinity. Both MHC class I and MHC class II show an exceptional structure while the peptide main chain is tightly bound and peptide side chains show less restrictive interactions. It is primarily the peptide side-chain contacts and conformational variability that ensures that the peptide-MHC complex presents an antigenically unique surface to T cell receptors.