page_title: Natural killer cell
section_title: NK cell receptors
hierarchical_section_title: Natural killer cell / NK cell receptors
caption_attribution_description: English: Cartoon representation of the molecular structure of protein registered with 1hq8 code. Deutsch: Grafik des Molekularstruktur von jenem Protein, das mit 1hq8 code registriert ist. Plattdüütsch: Grafik, de de Molekularstruktur von dat Protein wiest, dat mit’n Kood 1hq8 registreert is.
context_page_description: Natural killer cells, also known as NK cells or large granular lymphocytes, are a type of cytotoxic lymphocyte critical to the innate immune system. 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, acting at around 3 days after infection, and respond to tumor formation. Typically, immune cells detect the major histocompatibility complex presented on infected cell surfaces, triggering cytokine release, causing the death of the infected cell by lysis or apoptosis. NK cells are unique, however, as they have the ability to recognize and kill stressed cells in the absence of antibodies and MHC, allowing for a much faster immune reaction. 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 1. 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.
context_section_description: NK cell receptors can also be differentiated based on function. Natural cytotoxicity receptors directly induce apoptosis (cell death) after binding to Fas ligand that directly indicate infection of a cell. The MHC-independent receptors (described above) use an alternate pathway to induce apoptosis in infected cells. Natural killer cell activation is determined by the balance of inhibitory and activating receptor stimulation. For example, if the inhibitory receptor signaling is more prominent, then NK cell activity will be inhibited; similarly, if the activating signal is dominant, then NK cell activation will result.
  NK cell receptor types (with inhibitory, as well as some activating members) are differentiated by structure, with a few examples to follow:
caption: Protein structure of NKG2D
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