The monoclonal antibody (mAb) GB3 is specific for human Fc gamma receptor (FcγR)IIb. (a-c) Human B cell (Raji) or monocyte (U937) cell lines were stained with either the anti-FcγRIIb mAb GB3, the anti-FcγRIIa mAb IV.3 or the pan anti-FcγRII mAb KB61. Filled graphs represent the specific antibody and dotted graphs the isotype control antibody. Raji cells were clearly stained positive for FcγRIIb by the GB3 mAb, while U937 cells were negatively stained, thus demonstrating the FcγRIIb specificity of the GB3 mAb (a). Raji cells were, somewhat surprisingly, stained positive by IV.3 mAb and, as expected, so were the U937 cells (b). This indicates that the IV.3 mAb detects FcγRIIa but is also cross-reactive to FcγRIIb. Both cell lines were positively stained by the pan anti-FcγRII mAb, detecting both the a and b isoforms (c). (d) The epitope specificity of the GB3 mAb as demonstrated by ELISA. Serially diluted GB3 mAb show binding activity to recombinant sFcγRIIb whereas no reactivity to recombinant sFcγRIIa.