Functions as a coreceptor for the B-cell antigen receptor complex (BCR) on B-lymphocytes (PubMed:29523808). Decreases the threshold for activation of downstream signaling pathways and for triggering B-cell responses to antigens (PubMed:1373518, PubMed:16672701, PubMed:2463100). Activates signaling pathways that lead to the activation of phosphatidylinositol 3-kinase and the mobilization of intracellular Ca(2+) stores (PubMed:12387743, PubMed:16672701, PubMed:9317126, PubMed:9382888). 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:1373518, PubMed:2463100). Required for normal levels of serum immunoglobulins, and for production of high-affinity antibodies in response to antigen challenge (PubMed:12387743, PubMed:16672701, PubMed:9317126). {ECO:0000250|UniProtKB:P25918, ECO:0000269|PubMed:12387743, ECO:0000269|PubMed:1373518, ECO:0000269|PubMed:16672701, ECO:0000269|PubMed:2463100, ECO:0000269|PubMed:29523808, ECO:0000269|PubMed:9317126, ECO:0000269|PubMed:9382888}. This is the function of Ensembl gene identifier ENSG00000177455 (CD19, CD19 molecule).