The majority of polymorphic residues that distinguish different alleles of class I and class II molecules are located in positions that influence peptide binding or interaction with T-cell receptors. Structure of HLA-A*02:01:01 without (A) and with (B) the HA-1 minor H antigen bound in its peptide-binding groove, with the regions encoded by exons 2, 3, and 4 of the A*02:01:01 allele of HLA-A colored as in Figure 2. The specific residues in HLA-A*02:01:01 that are nonidentical with those in the HLA-A*03:01:01 molecule are indicated in yellow.