The B-cell antigen receptor (BCR) is composed of membrane immunoglobulin molecules (mIg) and associated Igalpha/Igbeta heterodimers (alpha/beta). The MIg subunits bind antigen and cause receptor aggregation, while the alpha/beta subunits transduce signals to the cell interior. Receptor aggregation rapidly activates Src family kinases, including Lyn, Btk and Fyn, initiating complex signaling cascades involving multiple adaptors, kinases, phosphatases, G-proteins and transcription factors. The complexity of BCR signaling permits many distinct outcomes, including proliferation, differentiation, apoptosis, survival and tolerance. The outcome of the response is determined by the maturation state of the cell, the affinity of the antibody-antigen interaction, the cellular environment and the nature of the antigen. Many other transmembrane receptors are known to modulate specific elements of BCR signaling. A few of these, including CD45 and CD19, are indicated above as closed rectangles.
