Figure 1.

Schematic representation of the main interactions occurring between normal natural killer (NK) cells (expressing both HLA class I-specific inhibitory receptors and activating receptors) and potential target cells. Normal tissues deliver inhibitory signals, which block NK cells (a). Normal stressed tissues deliver simultaneously both activatory and inhibitory signals, this also resulting in block of the lytic capacity of NK cells (b). The lack of inhibitory signals, by contrast, permits NK cells to kill their targets, activatory signals playing a facilitating role (c). Modified from Moretta et al. Immunology Today 2004.