Natural killer cell

Natural killer cells, also known as NK cells, are a type of cytotoxic lymphocyte critical to the innate immune system. They are a kind of large granular lymphocyte (LGL), belong to the rapidly expanding family of known innate lymphoid cells (ILC), and represent 5–20% of all circulating lymphocytes in humans. The role of NK cells is analogous to that of cytotoxic T cells in the vertebrate adaptive immune response. NK cells provide rapid responses to virus-infected cells, stressed cells, tumor cells, and other intracellular pathogens based on signals from several activating and inhibitory receptors. While cytotoxic T cells can only activate by detecting the antigen presented on major histocompatibility complex class I (MHC class I) molecules on infected cell surfaces, NK cells recognize and kill stressed cells that are lacking the MHC class I molecules. They were named "natural killers" because of the notion that they do not require activation to kill cells that are missing "self" markers of MHC class I. This role is especially important because harmful cells that are missing MHC I markers cannot be detected and destroyed by other immune cells, such as T lymphocyte cells. NK cells can be identified by the presence of CD56 and the absence of CD3 (CD56+, CD3−). NK cells differentiate from CD127+ common innate lymphoid progenitor, which is downstream of the common lymphoid progenitor from which B and T lymphocytes are also derived. NK cells are known to differentiate and mature in the bone marrow, lymph nodes, spleen, tonsils, and thymus, where they then enter into the circulation. NK cells differ from natural killer T cells (NKTs) phenotypically, by origin and by respective effector functions; often, NKT cell activity promotes NK cell activity by secreting interferon gamma. In contrast to NKT cells, NK cells do not express T-cell antigen receptors...