CD19 is a transmembrane glycoprotein that is delectively expressed on the cell surface of B-lymphocytes，where it activates intracellular signaling cascades involving both Ras and phosphatidylinositol 3-kinase pathways.Lymphocytes proliferate and differentiate in response to various concentrations of different antigens. The ability of the B cell to respond in a specific，yet sensitive manner to the various antigens is achieved with the use of low-affinity antigen receptors. This gene encodes a cell surface molecule which assembles with the antigen receptor of B lymphocytes in order to decrease the threshold for antigen receptor-dependent stimulation.