<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> <P> In addition to the knowledge that natural killer cells are effectors of innate immunity, recent research has uncovered information on both activating and inhibitory NK cell receptors which play important functional roles, including self tolerance and the sustaining of NK cell activity . NK cells also play a role in the adaptive immune response: numerous experiments have demonstrated their ability to readily adjust to the immediate environment and formulate antigen - specific immunological memory, fundamental for responding to secondary infections with the same antigen . The role of NK cells in both the innate and adaptive immune responses is becoming increasingly important in research using NK cell activity as a potential cancer therapy . </P> <P> NK cell receptors can also be differentiated based on function . NK Cells are not a subset of the T lymphocyte family . Natural cytotoxicity receptors directly induce apoptosis after binding to Fas ligand that directly indicate infection of a cell . The MHC - dependent 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 . </P>

What is the major function of a client's t helper cells