CD19 is a monomeric transmembrane glycoprotein expressed at relatively constant levels throughout B cell development from early pro-B/pre-B cells (i.e. B220+/CD43+/HSA+) through fully differentiated B cell stages. Terminally differentiated plasma cells do not express CD19. In humans, the CD19 molecule on the surface of mature B cells associates with CD21 (CR-2) and CD81 (TAPA-1), and this multimolecular complex synergizes with surface immunoglobulin to provide signal transduction and promote cellular activation. All splenic and peritoneal IgM+ cells of both B-1 and B-2 lineages are CD19+, with B-1 cells expressing higher levels of CD19 than B-2 cells in these sites. Studies with CD19-deficient mice have suggested that this molecule may not be required for normal generation and maturation of B cells in the bone marrow.