The P67.6 antibody reacts with CD33 (Siglec-3), a 67 kDa type 1 transmembrane sialoadhesin protein of the immunoglobulin superfamily. CD33 is expressed on committed myeloid progenitor cells, monocytes, granulocytes, dendritic cells, mast cells, resident macrophages, subsets of B cells, and activated T and NK cells. CD33 is a major marker used to differentiate myeloid from lymphoid or erythroid leukemias. In peripheral blood, it is expressed at high levels on monocytes and at lower levels on granulocytes. CD33 functions as a sialic acid-dependent cell adhesion molecule and mediates protein-glycan and protein-protein interactions between cells. It also has a putative role as an inhibitory receptor during the immune response.