This MAb reacts with the HLA-DRB1 antigen, a member of MHC class II molecules. It does not cross react with HLA-DP and HLA-DQ. It binds a conformational epitope on HLA-DR, which depends on the correct folding of the α/β heterodimer. This MAb has been reported to block mixed lymphocyte reactions. The L243 antibody recognizes a different epitope than the LN3 monoclonal antibody, and these antibodies do not cross-block binding to each other’s respective epitopes. HLA-DR is a heterodimeric cell surface glycoprotein comprised of a 36kD alpha (heavy) chain and a 28kD beta (light) chain. It is expressed on B-cells, activated T-cells, monocytes/macrophages, dendritic cells and other non-professional APCs. In conjunction with the CD3/TCR complex and CD4 molecules, HLA-DR is critical for efficient peptide presentation to CD4+ T cells. It is an excellent histiocytic marker in paraffin sections producing intense staining. True histiocytic neoplasms are similarly positive. HLA-DR antigens also occur on a variety of epithelial cells and their corresponding neoplastic counterparts.