Fc?RIIB (CD32B) is a low affinity, IgG Fc-binding receptor expressed on B cells, monocytes, macrophages, and dendritic cells (DCs). It is the inhibitory Fc receptor and signals through an immunoreceptor tyrosine-based inhibitory motif (ITIM) within its carboxy-terminal cytoplasmic tail. Binding of immune complexes to Fc?RIIB results in tyrosine phosphorylation of the ITIM motif at Tyr292 and recruitment of the phosphatase SHIP, which mediates inhibitory effects on immune cell activation. In this way, Fc?RIIB suppresses the effects of activating Fc-binding receptors. For example, mice deficient for Fc?RIIB have greater T cell and DC responses following injection of immune complexes. In addition, Fc?RIIB plays a role in B cell affinity maturation. Signaling through Fc?RIIB in the absence of signaling through the B cell receptor (BCR) is proapoptotic, while signaling through Fc?RIIB and the BCR simultaneously attenuates the apoptotic signal and results in selection of B cells with higher antigen affinity.