Aggregate NK function from 16 TCD-HSCT donor-recipient pairs. IFN-γ response to class I–negative 721.221 target cells was calculated relative to baseline for each inhibitory KIR in each of 16 TCD-HSCT donor-recipient pairs by the formula (KIR⁺IFN-γ⁺/KIR⁺)/(KIR⁻IFN-γ⁺/KIR⁻). The relative response of NK cells exclusively expressing specific inhibitory KIR was compared among the allograft donor, the patient at day +15 to 60, and the patient at days +100 to +200 after HSCT. (A) NK cells exclusively expressing inhibitory KIR to self-HLA from patients at days +15 to +60 are hyperresponsive compared with their donors (P = .004), normalizing to donor levels after day +100. (B) NK cells exclusively expressing inhibitory KIR to non-self HLA from patients at days +15 to +60 after HSCT are functionally competent (P < .001) compared with the donor, decreasing but still higher than donor by days +100 to +200 (P < .001). The paired Student t test was performed to compute the P value for these comparisons. NK cells expressing KIR for non-self HLA from days +15 to +60 NK cells are comparable in effector response to steady-state healthy donor NK cells expressing KIR for self-HLA (*).