Figure 2: NK cell activating receptors and signal transduction leading to killing targets. NK cells recognize target cells by various kinds of activating and inhibitory receptors. Activation uses common machinery that induces cytotoxicity for targets. The bindings of activating receptors with each ligand transduce through the Src family kinase- (SFK-) dependent phosphoinositide-3 kinase (PI3K) extracellular signal-regulated kinase 2 (ERK2) pathway and the phospholipase C c-Jun N-terminal kinase 1 (JNK1) pathway. Finally, degranulation is induced, by which perforin and granzymes in cytotoxic granules are released and work on target cells to induce apoptosis.