<P> Natural killer cells or NK cells are a type of cytotoxic lymphocyte critical to the innate immune system . The role NK cells play is analogous to that of cytotoxic T cells in the vertebrate adaptive immune response . NK cells provide rapid responses to viral - infected cells, acting at around 3 days after infection, and respond to tumor formation . Typically, immune cells detect major histocompatibility complex (MHC) presented on infected cell surfaces, triggering cytokine release, causing lysis or apoptosis . NK cells are unique, however, as they have the ability to recognize stressed cells in the absence of antibodies and MHC, allowing for a much faster immune reaction . They were named "natural killers" because of the initial 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 . </P> <P> NK cells (belonging to the group of innate lymphoid cells) are defined as large granular lymphocytes (LGL) and constitute the third kind of cells differentiated from the common lymphoid progenitor - generating B and T lymphocytes . 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 (TCR) or pan T marker CD3 or surface immunoglobulins (Ig) B cell receptors, but they usually express the surface markers CD16 (FcγRIII) and CD56 in humans, NK1. 1 or NK1. 2 in C57BL / 6 mice . The NKp46 cell surface marker constitutes, at the moment, another NK cell marker of preference being expressed in both humans, several strains of mice (including BALB / c mice) and in three common monkey species . </P>

What changes in mhc class i expression on a cell could activate nk cells
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