The human mononuclear phagocyte high-affinity Fc receptor, FcRI, defined by a monoclonal antibody, 10.1.

The properties of the mononuclear phagocyte (Mph) high-affinity Fc receptor, FcRI, were investigated using a novel monoclonal antibody (mAb) designated 10.1. This receptor was shown to be a protein of 71 kDa, presented chiefly on monocytes and the myeloid cell lines U937 and HL60. mAb 10.1 inhibited the binding to Mph of erythrocytes opsonized with rabbit IgG or human IgG3. It also blocked T cell proliferation induced by murine CD3 mAb of the IgG2a but not the IgG1 subclass. These results suggest that rabbit IgG, human IgG3 and murine IgG2a all bind to FcRI in a similar manner and that mAb 10.1 reacts with an epitope on FcRI near to the binding site for the Fc region of IgG. In addition, although it is well known that FcRI has a high affinity for both monomeric human IgG1 and IgG3, we show in this study that while erythrocytes opsonized with human IgG3 bind to Mph, equivalent cells opsonized with IgG1 surprisingly do not. These results define further the nature of the constraints on the interaction between Mph FcRI and particular IgGs.