IgG1 and IgG2b share the Fc receptor on mouse macrophages

Monoclonal IgG1 anti-SRBC has been used to study the binding of monomeric and aggregated IgG1 to Fc receptors on mouse macrophages. Aggregated IgG1 was found to bind to Fc receptors on two macrophage cell lines and on primary macrophages. It competes for binding with IgG2b and not IgG2a. Like the binding of IgG2b, the binding of IgG1 is unaltered at 4°C and is insensitive to trypsin or cytochalasin B. Antigen-bound IgG1 like IgG2b and IgG2a, mediated phagocytosis. Variant macrophage cell lines selected for the loss of phagocytosis through the IgG2b receptor no longer phagocytize IgG1 bearing SRBC. Monomeric IgG1 did not bind to either macrophage lines or primary macrophages. We conclude from these experiments that antigen-activated IgG1 binds to the same receptor as IgG2b.