An important goal of any serum screening procedure is to gain insight about the spectrum of HLA-specific antibodies, especially in highly sensitized patients. Each HLA molecule has multiple alloantigenic determinants (epitopes), which can be categorized as private or public. Such as epitopes also can be assiged by unique amino acid sequences. For example. HLA-A2 molecules carry a private epitope characterized by a histidine residue in position 74 in the α-1 helix and the cross-reactive epitope group of HLA-A2+A9+A28 molecules share a unique lysine residue in position 127 in the α-2 helix. Although highly sensitized patients have generally been exposed to many allogenic epitopes, most exhibit antibodies against a relative few, mostly public, epitopes, and their antibody specificity patterns remain relatively constant. A better understanding of the antibody specifity improves the selection of transplant donors with acceptable HLA mistmatch.