The W6/32 monoclonal antibody recognizes a monomorphic epitope expressed on native β2 microglobulin (β2m)-associated Major Histocompatibility Complex (MHC) class I molecules: Human Leucocyte Antigen (HLA)-A, HLA-B, and HLA-C (HLA-ABC). HLA class I molecules are heterodimers comprised of an ~40-45 kDa, highly polymorphic transmembrane α heavy chain, a type I glycoprotein that is noncovalently-associated with an invariant β2-microglobulin (β2m) light chain. The N-terminal extracellular region of the HLA class I heavy chain is comprised of three domains (α1, α2, and α3). The α1 and α2 domains form a closed antigen-binding groove that accommodates 8-10 aa-peptide antigens. β2m non-covalently associates with the α3 heavy chain domain and promotes HLA class I stability. The W6/32 antibody recognizes a conformational epitope on the HLA class I heavy chain. HLA Class I antigens are normally expressed on most nucleated cells. Their expression is upregulated on activated cells or cells responding to various agents including proinflammatory cytokines or mediators. Reduced HLA Class I expression is found on certain virus-infected or tumor cells. HLA class I antigens expressed on thymic epithelial cells are involved in the positive and negative selection of CD8+ T cell precursors which determines their TCR repertoire during T cell maturation. In the periphery, these HLA Class I antigens serve to either present endogenous antigens or cross-present exogenous antigens for the generation of effector and memory CD8+ T cell responses. Target cell HLA Class I antigens can serve as ligands for inhibitory receptors expressed on NK cells and CD8+ T cells and suppress their cytotoxic responses. Human HLA class I molecules play critical roles in cell-mediated immune responses and tumor surveillance as well as tolerance to self-antigens.