TC683 allows simultaneous detection and enumeration of FMC7-positive, CD23-positive and CD19-positive cells. The FMC7 antigen is expressed by B cells in normal peripheral blood. The antigen is also expressed by malignant cells in chronic lymphoproliferative disorders, including B-cell prolymphocytic leukemia, hairy cell leukemia, mantle cell lymphoma, follicle centre lymphoma, marginal zone lymphoma, mucosa-associated lymphoid tissue lymphoma, diffuse large cell lymphoma, Burkitt's lymphoma and Waldenström macroglobulinemia. CD23 is identical to the low affinity IgE receptor (FcεRII) found on B cells. CD23 is primarily expressed by B cells and monocytes and is also strongly expressed by EBV-transformed B lymphoblasts. In addition, CD23 is present on a large variety of other cells, such as T cells, eosinophils, platelets, Langerhans' cells and a subset of thymic epithelial cells. CD19 is a pan-B-cell antigen that is expressed by B lymphocytes at all stages of maturation excepting differentiated plasma cells. Antibodies to CD19 are considered essential for the initial evaluation of acute and chronic lymphoproliferative disorders.