This Program will delve into the concepts of HLA epitopes recognized by antibodies, their importance for evaluating mismatch acceptability and understanding antibody patterns of sensitized transplant patients. We will take a journey from the identification of Cross-Reactive Groups of antigens to the subsequent elucidation of epitopes from protein sequences and three-dimensional structures of HLA alleles that underpin their cross reactivity. The structural basis and definition of epitopes will be explained along with methods of determining immunogenicity and their application in antibody analysis and epitope matching.