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 (By similarity). Activates signaling pathways that lead to the activation of phosphatidylinositol 3-kinase and the mobilization of intracellular Ca(2+) stores (PubMed:9382888, PubMed:12387743, PubMed:20101619). Is not required for early steps during B cell differentiation in the blood marrow (PubMed:7542548, PubMed:7543183, PubMed:9317126). Required for normal differentiation of B-1 cells (PubMed:7542548, PubMed:7543183, PubMed:12387743). Required for normal B cell differentiation and proliferation in response to antigen challenges (PubMed:7542548, PubMed:9317126, PubMed:12387743). Required for normal levels of serum immunoglobulins, and for production of high-affinity antibodies in response to antigen challenge (PubMed:7542548, PubMed:7543183, PubMed:12387743). Interacts with CR2/CD21. Part of a complex composed of CD19, CR2/CD21, CD81 and IFITM1/CD225 in the membrane of mature B-cells. 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 when phosphorylated on Tyr-346 and/or Tyr-376. Interacts with PLCG2 when phosphorylated on Tyr-402. Interacts with LYN. Interacts (when tyrosine phosphorylated) with the regulatory p85 subunit of phosphatidylinositol 3-kinase (PIK3R1 or PIK3R2). Detected on B cells in spleen, bone marrow, thymus and lymph nodes (PubMed:12387743, PubMed:7542548, PubMed:20101619). Detected on peripheral blood lymphocytes (at protein level) (PubMed:7543183). Phosphorylated on tyrosine following B-cell activation (PubMed:20101619). Phosphorylated on tyrosine residues by LYN (By similarity). Tyrosine residues are phosphorylated sequentially after activation of the B cell receptor. Phosphorylation of Tyr-522 is extremely rapid, followed by phosphorylation at Tyr-402. In contrast, phosphorylation of Tyr-493 appears more slowly and is more transient, returning rapidly to basal levels (PubMed:20101619). Mutant mice are born at the expected Mendelian ratio, thrive and are fertile (PubMed:7542548). Mutant mice display normal differentiation and expansion of pro-B cells, pre-B cells, immature B cells and resting mature B cells in bone marrow (PubMed:7542548, PubMed:7543183, PubMed:9317126). Number and surface phenotype of conventional B cells in spleen, Peyer's patch and lymph nodes appear normal (PubMed:7543183). In contrast, the numbers of B-1 cells are decreased to 10-20% of the normal values in the peritoneal cavity, together with a severe reduction of serum IgM levels (PubMed:7542548, PubMed:7543183, PubMed:12387743). Likewise, mutant mice display severely reduced serum IgG1 and IgE levels (PubMed:7543183). Mutant mice display severely reduced B cell responses to antigens, with a strong decrease in the production of serum antibodies after an antigenic challenge, and defective production of high-affinity antibodies (PubMed:7543183).