FcγRIIb is an immune checkpoint46, 47 and its splice variants are potent modulators of ITAM‐dependent signaling (Figure 1c). This modulatory function occurs only when FcγRIIb is coaggregated with an ITAM signaling receptor. Thus, B‐cell activation by the binding of the antigen in the immune complex to the BCR is regulated by the simultaneous binding of the Fcs of the immune complex to FcγRIIb1 on the same cell. In innate leukocytes, the activating‐type FcR (i.e. FcγRI, FcγRIIa, FcγRIIc, FcγRIII) and the high‐affinity IgE receptor, FcεRI, and the IgA receptor, FcαRI, are all modulated by immune complex co‐engagement with FcγRIIb2. The inhibitory function contributes to the MOA of therapeutic antibodies that target cell‐activating molecules where the target cells also express the inhibitory FcγRIIbs such as the BCR (discussed later). Thus, B‐cell activation is modulated by the simulatenous binding of the antigen in the immune complex to the BCR and the binding of the Fcs, also in the immune complex, to FcγRIIB1 on the same cell.