Natural killer or NK cells belong to the granular lymphocyte of the innate immune arm. In particular, natural killer cells are designed to exert cytotoxicity against virus-infected cells and immunosurveillance of cancer cells. Various immunoreceptors on the NK cell surface are configured to sense any alteration of self cells caused by malignant transformation or viral infection, which are potentially nonspecific. In contrast, lymphocytes of the adaptive immune arm, B and T lymphocytes express one specific recognition receptor. NK cell receptors are grouped as inhibitory and activating receptors that are polymorphic and germline-encoded.
Inhibitory receptors override activating receptors when encountering normal cells. However, surface molecular changes by a viral infection or tumor formation stimulate the activating receptors. Upon activation, natural killer cell release granzyme, perforin, effector molecules of the TNF family, and Fas-ligand to induce apoptosis of the target cell. Besides, NK cells synthesize and release other cytokines and chemokines such as Il-10, gamma-interferon, GM-CSF to recruit other immune effector cells to the activated site. NK cells also exhibit antibody-dependent cell cytotoxicity when antibodies tag the target antigen or cell. Nowadays, interventions targetting the inhibiting-activating state of NK cells has been using in clinical therapies.