Figure 1. Effector responses of NK cells are regulated by inhibitory and activation receptors.

NK cells express receptors specific for ligands expressed on targets. Under normal circumstances, inhibitory receptors (in red) for MHC class I deliver signals that dominate over stimulation through activation receptors (in green) (middle pair of cells). When MHC class I is down-regulated, as in “missing-self,” activation receptors stimulate NK cell cytolysis of a target via exocytosis of granules (left). When a target is “stressed,” ligands for the NKG2D activation receptor are induced, as in “induced-self,” permitting NK cell activation by overcoming MHC class I-dependent inhibition (right). Depicted here is granule exocytosis against a target. NK cell responses also include cytokine production, regulated in the same way by signaling through a combination of activation and inhibitory receptors (not shown).