Receptor occupancy by lumiliximab in the presence of sCD23. Flow cytometric antibody assay used to measure the relative binding of lumiliximab to CD23. CD23-expressing cells are labeled with anti–CD23-PE antibodies that recognize an epitope different from that of lumiliximab. Cells are subsequently stained with lumiliximab-FITC. If the B-CLL cells have lumiliximab bound to CD23, lumiliximab-FITC, which is added during the assay period, will not bind to CD23, indicating that lumiliximab administered to the patient is still bound. FITC indicates fluorescein isothiocyanate.