In vivo–administered IFNα increases NK cell numbers and functionality but does not restore CD56^bright insufficiency. PBMCs were isolated from whole blood of patients 7 and 8 before and after initiation of IFNα2b treatment. (A) Patient (blue) or control (red) NK cell cytotoxicity was assayed against susceptible K562 target cells (left) before and (right) after treatment in the presence (dashed) or absence of IL-2 (solid). (B) PBMCs from whole blood were isolated and analyzed by FACS as in Figure 2 (top) before and (bottom) after treatment.