Natural killer cell

Natural killer cells (or NK cells) are a type of <a href="http://en.wikipedia.org/wiki/Cytotoxic" title="Cytotoxic" class="mw-redirect" >cytotoxic</a> <a href="http://en.wikipedia.org/wiki/Lymphocyte" title="Lymphocyte" >lymphocyte</a> critical to the <a href="http://en.wikipedia.org/wiki/Innate_immune_system" title="Innate immune system" >innate immune system</a>. The role <a href="http://en.wikipedia.org/wiki/NK_cells" title="NK cells" class="mw-redirect" >NK cells</a> play is analogous to that of <a href="http://en.wikipedia.org/wiki/Cytotoxic_T_cell" title="Cytotoxic T cell" >cytotoxic T cells</a> in the vertebrate <a href="http://en.wikipedia.org/wiki/Adaptive_immune_response" title="Adaptive immune response" class="mw-redirect" >adaptive immune response</a>. <a href="http://en.wikipedia.org/wiki/NK_cells" title="NK cells" class="mw-redirect" >NK cells</a> provide rapid responses to virally infected cells and respond to <a href="http://en.wikipedia.org/wiki/Tumor" title="Tumor" >tumor</a> formation, acting at around 3 days after <a href="http://en.wikipedia.org/wiki/Infection" title="Infection" >infection</a>.

Typically <a href="http://en.wikipedia.org/wiki/Immune_cells" title="Immune cells" class="mw-redirect" >immune cells</a> detect <a href="http://en.wikipedia.org/wiki/Major_histocompatibility_complex" title="Major histocompatibility complex" >MHC</a> presented on infected cell surfaces, triggering <a href="http://en.wikipedia.org/wiki/Cytokine" title="Cytokine" >cytokine</a> release causing <a href="http://en.wikipedia.org/wiki/Lysis" title="Lysis" >lysis</a> or <a href="http://en.wikipedia.org/wiki/Apoptosis" title="Apoptosis" >apoptosis</a>. <a href="http://en.wikipedia.org/wiki/NK_cells" title="NK cells" class="mw-redirect" >NK cells</a> are unique, however, as they have the ability to recognise stressed cells in the absence of <a href="http://en.wikipedia.org/wiki/Antibodies" title="Antibodies" class="mw-redirect" >antibodies</a> 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 in order to kill cells that are missing “self” markers of <a href="http://en.wikipedia.org/wiki/Major_histocompatibility_complex" title="Major histocompatibility complex" >major histocompatibility complex</a> (MHC) class 1 <a href="http://en.wikipedia.org/wiki/Natural_killer_cell#cite_note-Vivier2011-0" >[1]</a>.

NK cells are defined as large granular lymphocytes (LGL) and constitute the third kind of cells differentiated from the common <a href="http://en.wikipedia.org/wiki/Lymphoblast" title="Lymphoblast" >lymphoid progenitor</a>generating <a href="http://en.wikipedia.org/wiki/B_cell" title="B cell" >B</a> and <a href="http://en.wikipedia.org/wiki/T_cell" title="T cell" >T lymphocytes</a>.<a href="http://en.wikipedia.org/wiki/Natural_killer_cell#cite_note-Roitt-1" >[2]</a> NK cells are known to differentiate and mature in the <a href="http://en.wikipedia.org/wiki/Bone_marrow" title="Bone marrow" >bone marrow</a>, <a href="http://en.wikipedia.org/wiki/Lymph_node" title="Lymph node" >lymph node</a>, <a href="http://en.wikipedia.org/wiki/Spleen" title="Spleen" >spleen</a>, <a href="http://en.wikipedia.org/wiki/Tonsil" title="Tonsil" >tonsils</a> and <a href="http://en.wikipedia.org/wiki/Thymus" title="Thymus" >thymus</a> where they then enter into the circulation. <a href="http://en.wikipedia.org/wiki/Natural_killer_cell#cite_note-Lannello2008-2" >[3]</a>. NK cells differ from <a href="http://en.wikipedia.org/wiki/Natural_Killer_T_cell" title="Natural Killer T cell" >Natural Killer T cells</a> (NKT) phenotypically, by origin and by respective effector functions; often <a href="http://en.wikipedia.org/wiki/NKT_cell" title="NKT cell" class="mw-redirect" >NKT cell</a> activity promotes NK cell activity by secreting <a href="http://en.wikipedia.org/wiki/IFN%CE%B3" title="IFNγ" class="mw-redirect" >IFNγ</a>.

In contrast to NKT cells, NK cells do not express <a href="http://en.wikipedia.org/wiki/T-cell_antigen_receptors" title="T-cell antigen receptors" class="mw-redirect" >T-cell antigen receptors</a> (TCR) or Pan T marker <a href="http://en.wikipedia.org/wiki/CD3_(immunology)" title="CD3 (immunology)" >CD3</a> or surface <a href="http://en.wikipedia.org/wiki/Immunoglobulin" title="Immunoglobulin" class="mw-redirect" >immunoglobulins</a> (Ig) <a href="http://en.wikipedia.org/wiki/B_cell_receptor" title="B cell receptor" class="mw-redirect" >B cell receptors</a>, but they usually express the surface markers <a href="http://en.wikipedia.org/wiki/CD16" title="CD16" >CD16</a> (FcγRIII) and <a href="http://en.wikipedia.org/wiki/CD56" title="CD56" class="mw-redirect" >CD56</a> in humans, NK1.1 or NK1.2 in <a href="http://en.wikipedia.org/wiki/C57BL/6" title="C57BL/6" >C57BL/6</a> <a href="http://en.wikipedia.org/wiki/Mouse" title="Mouse" >mice</a>. Up to 80% of human NK cells also express <a href="http://en.wikipedia.org/wiki/CD8" title="CD8" >CD8</a>.

In addition to the knowledge that natural killer cells are effectors of <a href="http://en.wikipedia.org/wiki/Innate_immunity" title="Innate immunity" class="mw-redirect" >innate immunity</a>, recent research has uncovered information on both activating and inhibitory NK cell receptors which play important function roles including self tolerance and sustaining NK cell activity. NK cell also play a role in<a href="http://en.wikipedia.org/wiki/Adaptive_immune_response" title="Adaptive immune response" class="mw-redirect" >adaptive immune response</a> <a href="http://en.wikipedia.org/wiki/Natural_killer_cell#cite_note-Arina2007-3" >[4]</a>, numerous experiments have worked to demonstrate their ability to readily adjust to the immediate environment and formulate antigen-specific <a href="http://en.wikipedia.org/wiki/Immunological_memory" title="Immunological memory" class="mw-redirect" >immunological memory</a>, fundamental for responding to secondary infections with the same antigen. The ability for NK cells to act in both the innate and adaptive immune response is becoming increasingly important in research utilizing NK cell activity and potential cancer therapies.