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. Is not required for early steps during B cell differentiation in the blood marrow. Required for normal differentiation of B-1 cells. Required for normal B cell differentiation and proliferation in response to antigen challenges. Required for normal levels of serum immunoglobulins, and for production of high-affinity antibodies in response to antigen challenge (By similarity). 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 activated B cells. Interacts with VAV. Interacts with GRB2 and SOS when phosphorylated on Tyr-348 and/or Tyr-378. Interacts with PLCG2 when phosphorylated on Tyr-409. Interacts with LYN. Interacts (when tyrosine phosphorylated) with the regulatory p85 subunit of phosphatidylinositol 3-kinase (PIK3R1 or PIK3R2). Phosphorylated on tyrosine following B-cell activation (By similarity). Phosphorylated on tyrosine residues by LYN (By similarity). Tyrosine residues are phosphorylated sequentially after activation of the B cell receptor. Phosphorylation of Tyr-405 is extremely rapid (By similarity).