Engagement of individual activation receptors on resting NK cells does not induce degranulation. Resting NK cells were mixed with target cells as indicated. Where indicated, S2 cells were preincubated with a rabbit anti-S2 serum (+ IgG). Cells were incubated for 2 hours at 37°C, then stained with fluorochrome-conjugated anti-CD56 and anti-CD107a mAbs. The percentage increase of CD107a⁺ NK cells after incubation with target cells relative to CD107a⁺ NK cells upon incubation of NK cells alone (ΔCD107a⁺) is presented as the mean of 8 donors. represents target cells as indicated, whereas ■ represents target cells coexpressing ICAM-1 in addition to the other ligands, as indicated. Bars denote SD.