ABSTRACT
The purpose of the immune system is to recognize foreign antigen and tailor the type, strength, and longevity of the response to the nature of the stimulus. B-cell activation and differentiation is dependent upon engagement of the B-cell antigen receptor (BCR, or surface immunoglobulin, sIg). The BCR complex is composed of the immunoglobulin molecule found in noncovalent association with the Igoα and IgB proteins (CD79a, CD79b), which provide the signaling capability for sIg. Antigen engagement of the BCR results in the rapid phosphorylation of the Igα and Igβ components [1], and of the src and syk families of tyrosine kinases. Activation of these kinases results in tyrosine phosphorylation and activation of multiple parallel effector pathways that include the phospholipase Cγ (PLCγ), Ras/MAP kinase, and phosphatidylinositol-3-kinase (PI3K) pathways. Long thought to be the only component required to mount an immune response, it is now obvious that while the sIg molecule is critically involved, it is not sufficient for regulating the strength and maturation of the response. We now know that the magnitude of the B-cell response to antigen is governed by the collective input of accessory molecules and co-receptors that can augment or inhibit signals transduced through sIg. The contents of this chapter will focus on the contribution of the co-receptor molecules, CD19, CD21, CD81, CD22, and FcγRIIB; with a brief statement on the most recently identified co-receptor, PIR-B.
