Activation receptor coengagement on resting NK cells enhances CD16-induced 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, thereafter stained with fluorochrome-conjugated anti-CD56 and anti-CD107a mAbs. (A) The percentage of ΔCD107a⁺ CD56^dim NK cells is presented and values are representative of more than 3 independent experiments.