IFNG and influenza: To determine the effect of a low concentration of exogenous IL-15 on the frequency and kinetics of NK cell responses to influenza, IFN-γ, CD107a, and CD25 were measured at 6, 9, and 18 h after stimulation of human PBMCs with H3N2 in the presence or absence of 0.75 ng/ml recombinant human IL-15.