Figure 1.

B cells express two types of Fc receptors of opposing functions. FcμR promotes B cell activation via interacting with the BCR and potentiating BCR signaling. In contrast, B cells express FcγRIIB, which inhibits B cell activation upon binding to immune complexes containing IgG and the cognate Ag, which then results in colligation of FcγRIIB and the BCR. Therefore, B cells express two types of Fc receptors of opposing functions. It is suggested that these two receptors function in a spatial-temporal manner to positively and negatively regulate B cell activation during humoral immune responses (see Figure 2).