Model of B-cell inhibition by IgG and B-cell stimulation through IgM. MV is bound by the BCR of a MV-specific B cell. If MV-specific IgG binds to MV, the constant region will be bound by the receptor for the constant region (Fc) of IgG (which is FcγRIIB). FcγRIIB is the only Fc receptor on B cells and does not bind other immunoglobulins, such as IgM or IgA. After juxtaposition of the BCR and FcγRIIB, the tyrosine-based inhibitory motif of FcγRIIB is in close proximity of the tyrosine-based activation motif of BCR and delivers a negative signal. If MV-specific IgM binds to MV, it also binds via C3d to CD21 (complement receptor 2), which is part of the positive signaling CD21/CD19/CD81/Leu-13 complex. The opsonin C3d does not bind to IgG.