Functions as coreceptor for the B-cell antigen receptor complex (BCR) on B-lymphocytes. Decreases the threshold for activation of downstream signaling pathways and for triggering B-cell responses to antigens (PubMed:
2463100, PubMed:
1373518, PubMed:
16672701). Activates signaling pathways that lead to the activation of phosphatidylinositol 3-kinase and the mobilization of intracellular Ca(2+) stores (PubMed:
9382888, PubMed:
9317126, PubMed:
12387743, PubMed:
16672701). Is not required for early steps during B cell differentiation in the blood marrow (PubMed:
9317126). Required for normal differentiation of B-1 cells (By similarity). Required for normal B cell differentiation and proliferation in response to antigen challenges (PubMed:
2463100, PubMed:
1373518). Required for normal levels of serum immunoglobulins, and for production of high-affinity antibodies in response to antigen challenge (PubMed:
9317126, PubMed:
12387743, PubMed:
16672701).