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==Complement system==
{{main|Complement system}}
The [[complement system]] is a [[biochemical cascade]] of the immune system that helps, or “complements”"complements", the ability of antibodies to clear pathogens or mark them for destruction by other cells. The cascade is composed of many plasma proteins, synthesized in the [[liver]], primarily by [[hepatocytes]]. The proteins work together to:
* trigger the recruitment of inflammatory cells
* "tag" pathogens for destruction by other cells by [[Opsonin|''opsonizing'']], or coating, the surface of the pathogen
====Macrophages====
{{main|Macrophages}}
Macrophages, from the Greek, meaning "large eaters", are large phagocytic leukocytes, which are able to move beyond the vascular system by migrating through the walls of [[capillary]] vessels and entering the areas between cells in pursuit of invading pathogens. In tissues, organ-specific macrophages are differentiated from phagocytic cells present in the blood called [[monocyte]]s. Macrophages are the most efficient phagocytes and can phagocytose substantial numbers of bacteria or other cells or microbes.<ref name="Janeway"/> The binding of bacterial molecules to receptors on the surface of a macrophage triggers it to engulf and destroy the bacteria through the generation of a “"[[respiratory burst]]”", causing the release of [[reactive oxygen species]]. Pathogens also stimulate the macrophage to produce chemokines, which summon other cells to the site of infection.<ref name="Janeway"/>
====Neutrophils====
===Natural killer cells===
{{main|Natural killer cell}}
[[Natural killer cells]] (NK cells) do not directly attack invading microbes. Rather, NK cells destroy compromised host cells, such as [[tumor]] cells or virus-infected cells, recognizing such cells by a condition known as "missing self.". This term describes cells with abnormally low levels of a cell-surface marker called MHC I ([[major histocompatibility complex]]) - a situation that can arise in viral infections of host cells.<ref name="Janeway6">{{cite book| vauthors = Janeway C |title = Immunobiology |edition = 6th|year = 2005|publisher = Garland Science|isbn = 0-443-07310-4}}</ref> They were named "natural killer" because of the initial notion that they do not require activation in order to kill cells that are "missing self.". The MHC makeup on the surface of damaged cells is altered and the NK cells become activated by recognizing this. Normal body cells are not recognized and attacked by NK cells because they express intact self MHC antigens. Those MHC antigens are recognized by killer cell [[immunoglobulin]] receptors (KIR) that slow the reaction of NK cells. The [[NK-92]] cell line does not express KIR and is developed for tumor therapy.<ref>{{cite journal | vauthors = Arai S, Meagher R, Swearingen M, Myint H, Rich E, Martinson J, Klingemann H | title = Infusion of the allogeneic cell line NK-92 in patients with advanced renal cell cancer or melanoma: a phase I trial | journal = Cytotherapy | volume = 10 | issue = 6 | pages = 625–632 | year = 2008 | pmid = 18836917 | doi = 10.1080/14653240802301872 }}</ref><ref>{{cite journal | vauthors = Tonn T, Becker S, Esser R, Schwabe D, Seifried E | title = Cellular immunotherapy of malignancies using the clonal natural killer cell line NK-92 | journal = Journal of Hematotherapy & Stem Cell Research | volume = 10 | issue = 4 | pages = 535–544 | date = August 2001 | pmid = 11522236 | doi = 10.1089/15258160152509145 }}</ref><ref name="pmid8152260">{{cite journal | vauthors = Gong JH, Maki G, Klingemann HG | title = Characterization of a human cell line (NK-92) with phenotypical and functional characteristics of activated natural killer cells | journal = Leukemia | volume = 8 | issue = 4 | pages = 652–658 | date = April 1994 | pmid = 8152260 }}</ref><ref>{{cite book | vauthors = Klingemann HG | chapter = Development and testing of NK cell lines | veditors = Lotze MT, Thompson AW | title = Natural killer cells - Basic Science and Clinical applications | date = 2010 | pages = 169–175 }}</ref>
===γδ T cells===
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