This MAb reacts with a CD32 (FcgRII) epitope distinct from that defined by MAb 8.26 and the epitope overlaps with that of MAb 7.30 (cluster 4). It displays a stronger reaction with Daudi than with U937 cells. The epitope is located in domain 2 of FcgRIIa. Its Fab'2 fragments block immune complex binding. CD32 (FcRII) is a type 1 transmembrane glycoprotein that mediates several functions including phagocytosis, cytotoxicity, and immunomodulation as well as platelet aggregation. Three genes (A, B, and C) encode CD32 and at least 6 isoforms are generated via alternative mRNA splicing, i.e., IIa1, IIa2, IIb1, IIb2, IIb3 and IIc. Monocytes/macrophages, placental trophoblasts and endothelial cells express all isoforms. In addition, the IIb isoform is expressed by B cells, and the IIa isoform by platelets, granulocytes and, weakly, by B cells. NK cells and neutrophils express Isoform IIc. CD32 binds weakly to the Fc region of monomeric IgG but more strongly to IgG aggregates and immune complexes.