Description: The HIM3-4 monoclonal antibody reacts with human CD33, a 67 kDa member of the sialoadhesion family. CD33 is expressed by myelomonocytic precursors, monocytes, mast cells, and granulocytes. Hematopoietic stem cells and lymphocytes do not express this antigen. CD33 plays a role in sialic-acid dependent cell adhesion.