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[[File:Innate immune system.png|thumb|Innate immune system]]
The '''innate immune system''' or '''nonspecific immune system'''<ref>{{Cite web|title=Immune response: MedlinePlus Medical Encyclopedia|url=https://medlineplus.gov/ency/article/000821.htm|access-date=2021-11-07|website=medlineplus.gov|language=en}}</ref> is one of the two main immunity strategies in [[vertebrate]]s (the other being the [[adaptive immune system]])
The major functions of the innate immune system are to:
* recruit immune cells to infection sites by producing chemical factors, including chemical mediators called [[cytokine]]s
* activate the [[complement cascade]] to identify [[bacteria]], activate cells, and promote clearance of [[immune complex|antibody complexes]] or dead cells
* identify and remove foreign substances present in organs, tissues, blood and [[lymph]], by specialized [[white blood
* activate the [[adaptive immune system]] through [[antigen presentation]]
* act as a physical and chemical barrier to infectious agents; via physical measures such as skin and mucus, and chemical measures such as [[clotting factor]]s and [[host defence peptide]]s.
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{{See also|Nonspecific immune cell#Cells of the innate immune system|l1=Cells of the innate immune system}}
[[File:SEM blood cells.jpg|thumb|right|230px|A [[scanning electron microscope]] image of normal circulating human blood. One can see red blood cells, several knobby white blood cells including [[#Cells of the Adaptive Immune System|lymphocytes]], a [[#Macrophages|monocyte]], a [[#Neutrophils|neutrophil]], and many small disc-shape [[platelet]]s.]]
White blood cells (WBCs) are also known as [[leukocyte]]s. Most leukocytes differ from other cells of the body in that they are not tightly associated with a particular organ or tissue; thus, their function is similar to that of independent, single-cell organisms. Most leukocytes are able to move freely and interact with and capture cellular debris, foreign particles, and invading microorganisms (although [[macrophage]]s, [[mast cell]]s, and [[dendritic cell]]s are less mobile). Unlike many other cells, most innate immune leukocytes cannot divide or reproduce on their own, but are the products of multipotent [[hematopoietic stem cell]]s present in [[bone marrow]].<ref name="Monga">{{cite journal | vauthors = Monga I, Kaur K, Dhanda S| title = Revisiting hematopoiesis: applications of the bulk and single-cell transcriptomics dissecting transcriptional heterogeneity in hematopoietic stem cells | journal = Briefings in Functional Genomics | volume = 21 | issue = 3 | pages = 159–176 | date = March 2022 | pmid = 35265979 | doi = 10.1093/bfgp/elac002}}</ref><ref name="Alberts">{{cite book| vauthors =Alberts B, Johnson A, Lewis J, Raff M, Roberts K, Walters P| title = Molecular Biology of the Cell | edition = Fourth | url = https://www.ncbi.nlm.nih.gov/books/
The innate leukocytes include: [[natural killer cells]], mast cells, [[eosinophils]], [[basophils]]; and the [[phagocytic cells]] include [[macrophages]], [[neutrophils]], and dendritic cells, and function within the immune system by identifying and eliminating pathogens that might cause infection.<ref name="Janeway" />
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====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"/> There are also resident macrophages in many tissues including mucous membranes, liver, lungs, and skin (where they are often called [[Langerhans cell]]s).
====Neutrophils====
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====Dendritic cells====
{{main|Dendritic cell}}
Dendritic cells (DCs) are phagocytic cells present in tissues that are in contact with the external environment, mainly the [[Human skin|skin]]
[[File:PBEosinophil.jpg|thumb|left|120px|An eosinophil]]
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* [[measles]]
* [[rhinovirus]]
|| yes || yes<ref>{{cite journal | vauthors = Agrawal P, Nawadkar R, Ojha H, Kumar J, Sahu A | title = Complement Evasion Strategies of Viruses: An Overview | journal = Frontiers in Microbiology | volume = 8 |
|-
| rowspan="2" | Intracellular '''[[bacteria]]''' ||
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* ''[[Histoplasma]]''
* ''[[Cryptococcus (fungus)|Cryptococcus]]''
||
|-
|}
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===Plants===
{{Main|Plant disease resistance#Immune system}}
Members of every class of pathogen that infect humans also infect plants. Although the exact pathogenic species vary with the infected species, bacteria, fungi, viruses, nematodes, and insects can all cause [[Phytopathology|plant disease]]. As with animals, plants attacked by insects or other pathogens use a set of complex [[metabolic]] responses that lead to the formation of defensive chemical compounds that fight infection or make the plant less attractive to insects and other [[herbivore]]s.<ref name="Plant">{{cite web | vauthors = Schneider D | date = 2005 | url = http://cmgm.stanford.edu/micro/Schneider-lab/Innate%20immunity%
Like invertebrates, plants neither generate antibody or T-cell responses nor possess mobile cells that detect and attack pathogens. In addition, in case of infection, parts of some plants are treated as disposable and replaceable, in ways that few animals can. Walling off or discarding a part of a plant helps stop infection spread.<ref name= Plant/>
Most plant immune responses involve systemic chemical signals sent throughout a plant. Plants use PRRs to recognize conserved microbial signatures. This recognition triggers an immune response. The first plant receptors of conserved microbial signatures were identified in rice ([[XA21]], 1995)<ref>{{cite journal | vauthors = Song WY, Wang GL, Chen LL, Kim HS, Pi LY, Holsten T, Gardner J, Wang B, Zhai WX, Zhu LH, Fauquet C, Ronald P | display-authors = 6 | title = A receptor kinase-like protein encoded by the rice disease resistance gene, Xa21 | journal = Science | volume = 270 | issue = 5243 | pages = 1804–1806 | date = December 1995 | pmid = 8525370 | doi = 10.1126/science.270.5243.1804 | s2cid = 10548988 | bibcode = 1995Sci...270.1804S | url = https://escholarship.org/uc/item/4x0247kj | url-access = subscription }}</ref><ref>{{cite journal | vauthors = Ronald PC, Beutler B | title = Plant and animal sensors of conserved microbial signatures | journal = Science | volume = 330 | issue = 6007 | pages = 1061–1064 | date = November 2010 | pmid = 21097929 | doi = 10.1126/science.1189468 | s2cid = 18311102 | bibcode = 2010Sci...330.1061R | url = https://escholarship.org/uc/item/9q96r8gz }}</ref> and in ''[[Arabidopsis]]'' ([[FLS2]], 2000).<ref>{{cite journal | vauthors = Gómez-Gómez L, Boller T | title = FLS2: an LRR receptor-like kinase involved in the perception of the bacterial elicitor flagellin in Arabidopsis | journal = Molecular Cell | volume = 5 | issue = 6 | pages = 1003–1011 | date = June 2000 | pmid = 10911994 | doi = 10.1016/S1097-2765(00)80265-8 | doi-access = free }}</ref> Plants also carry immune receptors that recognize variable pathogen effectors. These include the NBS-LRR class of proteins. When a part of a plant becomes infected with a microbial or viral pathogen, in case of an incompatible interaction triggered by specific [[elicitors]], the plant produces a localized [[hypersensitive response]] (HR), in which cells at the site of infection undergo rapid apoptosis to prevent spread to other parts of the plant. HR has some similarities to animal [[pyroptosis]], such as a requirement of [[caspase]]-1-like proteolytic activity of [[VPEγ]], a [[cysteine protease]] that regulates cell disassembly during cell death.<ref>{{cite journal | vauthors = Rojo E, Martín R, Carter C, Zouhar J, Pan S, Plotnikova J, Jin H, Paneque M, Sánchez-Serrano JJ, Baker B, Ausubel FM, Raikhel NV | display-authors = 6 | title = VPEgamma exhibits a caspase-like activity that contributes to defense against pathogens | journal = Current Biology | volume = 14 | issue = 21 | pages = 1897–1906 | date = November 2004 | pmid = 15530390 | doi = 10.1016/j.cub.2004.09.056 | s2cid = 3231431 | doi-access = free | bibcode = 2004CBio...14.1897R }}</ref>
"Resistance" (R) proteins, encoded by [[R gene]]s, are widely present in plants and detect pathogens. These proteins contain domains similar to the [[NOD-like receptor|NOD Like Receptors]] and TLRs. [[Systemic acquired resistance]] (SAR) is a type of defensive response that renders the entire plant resistant to a broad spectrum of infectious agents.<ref>[[Chitosan#Agricultural and horticultural use]]</ref> SAR involves the production of [[chemical messenger (disambiguation)|chemical messenger]]s, such as [[salicylic acid]] or [[jasmonic acid]]. Some of these travel through the plant and signal other cells to produce defensive compounds to protect uninfected parts, e.g., leaves.<ref>{{cite journal | vauthors = Linden JC, Stoner RJ, Knutson KW, Gardner-Hughes CA | title = Organic disease control elicitors. | journal = Agro Food Industry Hi-Tech | date = 2000 | volume = 11 | issue = 5 | pages = 32–34 | url=http://www.yeacrops.com/Crop%20Protection%20Article.pdf| url-status=dead| archive-url= https://web.archive.org/web/20070706111024/http://www.yeacrops.com/Crop%20Protection%20Article.pdf| archive-date=6 July 2007| df=dmy-all }}</ref> Salicylic acid itself, although indispensable for expression of SAR, is not the translocated signal responsible for the systemic response. Recent evidence indicates a role for [[jasmonates]] in transmission of the signal to distal portions of the plant. [[RNA interference|RNA silencing]] mechanisms are important in the plant systemic response, as they can block virus replication.<ref>{{cite journal | vauthors = Baulcombe D | title = RNA silencing in plants | journal = Nature | volume = 431 | issue = 7006 | pages = 356–363 | date = September 2004 | pmid = 15372043 | doi = 10.1038/nature02874 | s2cid = 4421274 | bibcode = 2004Natur.431..356B }}</ref> The ''[[jasmonic acid]] response'' is stimulated in leaves damaged by insects, and involves the production of [[methyl jasmonate]].<ref name= Plant/>
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* [[NOD-like receptor]]
* [[Endothelial cell tropism]]
* [[Selnoflast]]
== References ==
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