Antigen-specific immune responses are largely targeted to eliminate pathogen proteins. However, these pathways can also eliminate transfused cells through recognition of variant antigens expressed on the surface of donor red cells and platelets. While some of the peptide moieties that are recognized by antibodies to blood group antigens have been characterized, we have little information on the identity of antigenic peptides recognized by Tcells that drive the immune response against transfused cells. In this session, the speakers will review and discuss our recent understanding of the initial steps in molecular recognition of antigens through T-cell MHC-peptide interactions, and the nature of differences between auto- and allorecognition. The session will also describe recent advances in identification of T-cell epitopes to blood group antigens and how this information can be exploited diagnostically to prevent immunological responses to transfused cells.