Lymphocytes proliferate and differentiate in response to various concentration 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. CD19 is a pan B-cell marker expressed in a wide range of maturational stages including pre-B cells. CD19 is a member of the immunoglobulin superfamily and has two Ig like domains. The CD19 molecule is expressed on 100% of the peripheral B cells as defined by expression of kappa or lambda light chains. CD19 appears to be expressed on myeloid leukemia cells, particularly those of monocytic lineage. Leukemia phenotype studies have demonstrated that the earliest and broadest B cell restricted antigen is the CD19 antigen. The receptor for CD19 is an important functional regulator of normal and malignant B cell proliferation, and is expressed in all B cell precursor leukemias. 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. CD19 is a cell surface molecule which assembles with the antigen receptor of B lymphocytes in order to decrease the threshold for antigen receptor-dependent stimulation.