A higher degree of flexibility is observed in allorestricted as compared to self-restricted antigen recognition. (A) Relative recognition of peptide Melan-A 26-35 or of single A-substituted variants was assessed in a ⁵¹Cr release assay on T2 target cells in the presence of increasing concentrations of the indicated peptides. (B) The nanomolar concentration of peptide giving 50% maximal activity was determined from the titration curve of each peptide. Recognition of single amino acid substituted variants was calculated relative to that of the parental Melan-A 26-35 peptide. For each variant, the resulting value was then divided by the corresponding relative HLA-A2 binding value determined previously in a functional competition assay.