HLA sequence polymorphisms that are recognized by antibodies (serologic method) are termed antigens, whereas those that can be identified only by DNA-based typing methods are termed alleles. Any given HLA antigen may be encoded by a family of HLA alleles with similar nucleotide sequences. Transplant donor and recipient pairs with different HLA antigens always have different alleles (antigen mismatched), and pairs with the same allele always have the same antigen (matched). Some donors and recipients with the same HLA antigen have different alleles (alleles mismatched). With few exceptions, HLA alleles mismatched are characterized by amino acid substitutions in the region of the HLA molecule that bind peptides for presentation to T cells, whereas HLA antigen mismatches are characterized by amino acid substitutions relevant to both peptide binding and contact with T cells.