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). Interacts with CR2/CD21 (PubMed:1702139). Part of a complex composed of CD19, CR2/CD21, CD81 and IFITM1/CD225 in the membrane of mature B-cells (PubMed:1383329). Interacts directly with CD81 (via the second extracellular domain); this interaction is initiated early during biosynthesis in the ER/pre-Golgi compartments and is essential for trafficking and compartmentalization of CD19 receptor on the cell surface of activated B cells (PubMed:16449649). Interacts with VAV (PubMed:10706702). Interacts with GRB2 and SOS when phosphorylated on Tyr-348 and/or Tyr-378. Interacts with PLCG2 when phosphorylated on Tyr-409 (PubMed:10706702). Interacts with LYN (PubMed:7687428). Interacts (when tyrosine phosphorylated) with the regulatory p85 subunit of phosphatidylinositol 3-kinase (PIK3R1 or PIK3R2) (PubMed:7684160). Detected on marginal zone and germinal center B cells in lymph nodes (PubMed:2463100). Detected on blood B cells (at protein level) (PubMed:2463100, PubMed:16672701). Phosphorylated on tyrosine following B-cell activation (PubMed:7684160, PubMed:7687539, PubMed:10706702, PubMed:12387743). Phosphorylated on tyrosine residues by LYN (PubMed:7687428). Tyrosine residues are phosphorylated sequentially after activation of the B cell receptor. Phosphorylation of Tyr-531 is extremely rapid, followed by phosphorylation at Tyr-409. In contrast, phosphorylation of Tyr-500 appears more slowly and is more transient, returning rapidly to basal levels (By similarity). The disease is caused by variants affecting the gene represented in this entry. CD19 mutation db