Before revision:  Although NK cells find and attack K562 cells with high efficiency, we show that the attacks are consistent with a target-blind random walk. NK cells achieve their high attack efficiency by combining high migration speeds with high degrees of directional persistence .
After revision:  Although NK cells find and attack K562 and NK cells for one of the donors. By contrast, we find attractive interactions between NK cells and an IL-15-secreting variant of K562 tumor cells .