The monoclonal antibody BB7.2 specifically binds to the α subunit of the human leukocyte antigen-A2 (HLA-A2), a class I molecule of the major histocompatibility complex (MHC). The MHC gene locus encodes a group of highly polymorphic, cell-surface proteins that play a broad role in the immune response to protein antigens. MHC molecules bind and present small antigenic protein fragments to antigen-specific receptors expressed by T cells (TCR). Human (human leukocyte antigen/HLA) MHC molecules are comprised of two major classes, MHC class I and class II. Functionally, class I MHC molecules bind peptides derived from intracellular antigens (eg, viral and some bacterial antigens) which are specifically recognized by CD8+ T cells. Class II MHC molecules bind antigens derived from pathogens multiplying in intracellular vesicles and ingested extracellular bacteria, both of which are recognized by CD4+ T cells. TCR recognize processed peptides bound to the MHC as well as regions of the MHC molecule itself. CD4 and CD8 accessory molecules strengthen the formation of the TCR-MHC complex through their interaction with non-polymorphic regions of the MHC molecule.