Editing Immune system (section)

===Evolution of the immune system===

It is likely that a multicomponent, adaptive immune system arose with the first [[vertebrate]]s, as [[invertebrate]]s do not generate lymphocytes or an antibody-based humoral response.<ref name="pmid19997068" >{{cite journal |vauthors=Flajnik MF, Kasahara M |title=Origin and evolution of the adaptive immune system: genetic events and selective pressures |journal=Nature Reviews. Genetics |volume=11 |issue=1 |pages=47–59 |date=January 2010 |pmid=19997068 |pmc=3805090 |doi=10.1038/nrg2703 }}</ref> Immune systems evolved in [[deuterostome]]s as shown in the cladogram.<ref name="pmid19997068"/>

{{clade|style=font-size:95%;line-height:110%;
|label1=[[Deuterostome]]s
|sublabel1='''[[innate immunity]]'''
|1={{clade
   |1=[[Echinoderm]]s, [[hemichordate]]s, [[cephalochordate]]s, [[urochordate]]s
   |label2=[[Vertebrates]]
   |2={{clade
      |sublabel1='''[[variable lymphocyte receptor|VLR adaptive immunity]]'''
      |1=[[Agnatha|Jawless fishes]]
      |sublabel2='''[[V(D)J recombination|V(D)J adaptive immunity]]'''
      |2=[[Osteichthyes|Jawed fishes and tetrapods]]
      }}
   }}
}}

Many species, however, use mechanisms that appear to be precursors of these aspects of vertebrate immunity. Immune systems appear even in the structurally simplest forms of life, with bacteria using a unique defense mechanism, called the [[restriction modification system]] to protect themselves from viral pathogens, called [[bacteriophage]]s.<ref>{{cite journal | vauthors = Bickle TA, Krüger DH | title = Biology of DNA restriction | journal = Microbiological Reviews | volume = 57 | issue = 2 | pages = 434–50 | date = Jun 1993 | pmid = 8336674 | pmc = 372918 | doi = 10.1128/MMBR.57.2.434-450.1993 }}</ref> [[Prokaryote]]s ([[bacteria]] and [[archea]]) also possess acquired immunity, through a system that uses [[CRISPR]] sequences to retain fragments of the genomes of phage that they have come into contact with in the past, which allows them to block virus replication through a form of [[RNA interference]].<ref>{{cite journal | vauthors = Barrangou R, Fremaux C, Deveau H, Richards M, Boyaval P, Moineau S, Romero DA, Horvath P | title = CRISPR provides acquired resistance against viruses in prokaryotes | journal = Science | volume = 315 | issue = 5819 | pages = 1709–12 | date = Mar 2007 | pmid = 17379808 | doi = 10.1126/science.1138140 | bibcode = 2007Sci...315.1709B | hdl = 20.500.11794/38902 | s2cid = 3888761 | hdl-access = free }}</ref><ref>{{cite journal | vauthors = Brouns SJ, Jore MM, Lundgren M, Westra ER, Slijkhuis RJ, Snijders AP, Dickman MJ, Makarova KS, Koonin EV, van der Oost J | title = Small CRISPR RNAs guide antiviral defense in prokaryotes | journal = Science | volume = 321 | issue = 5891 | pages = 960–64 | date = Aug 2008 | pmid = 18703739 | pmc = 5898235 | doi = 10.1126/science.1159689 | bibcode = 2008Sci...321..960B }}</ref> Prokaryotes also possess other defense mechanisms.<ref>{{cite journal | vauthors = Hille F, Charpentier E | title = CRISPR-Cas: biology, mechanisms and relevance | journal = Philosophical Transactions of the Royal Society of London. Series B, Biological Sciences | volume = 371 | issue = 1707 | pages = 20150496 | date = November 2016 | pmid = 27672148 | pmc = 5052741 | doi = 10.1098/rstb.2015.0496 }}</ref><ref>{{cite journal | vauthors = Koonin EV | title = Evolution of RNA- and DNA-guided antivirus defense systems in prokaryotes and eukaryotes: common ancestry vs convergence | journal = Biology Direct | volume = 12 | issue = 1 | pages = 5 | date = February 2017 | pmid = 28187792 | pmc = 5303251 | doi = 10.1186/s13062-017-0177-2 | doi-access = free }}</ref> Offensive elements of the immune systems are also present in [[protist|unicellular eukaryotes]], but studies of their roles in defense are few.<ref>{{cite journal | vauthors = Bayne CJ | year = 2003 | title = Origins and evolutionary relationships between the innate and adaptive arms of immune systems | journal = Integr. Comp. Biol. | volume = 43 | issue = 2| pages = 293–99 | pmid = 21680436 | doi=10.1093/icb/43.2.293| doi-access = free }}</ref>

[[Pattern recognition receptor]]s are proteins used by nearly all organisms to identify molecules associated with pathogens. [[Antimicrobial peptides]] called [[defensin]]s are an evolutionarily conserved component of the innate immune response found in all animals and plants, and represent the main form of invertebrate systemic immunity.<ref name="pmid19997068" /> The [[complement system]] and phagocytic cells are also used by most forms of invertebrate life. [[Ribonuclease]]s and the [[RNA interference]] pathway are conserved across all [[eukaryote]]s, and are thought to play a role in the immune response to viruses.<ref>{{cite journal | vauthors = Stram Y, Kuzntzova L | title = Inhibition of viruses by RNA interference | journal = Virus Genes | volume = 32 | issue = 3 | pages = 299–306 | date = Jun 2006 | pmid = 16732482 | doi = 10.1007/s11262-005-6914-0 | pmc = 7088519 }}</ref>

Unlike animals, plants lack phagocytic cells, but many plant immune responses involve systemic chemical signals that are sent through a plant.<ref name= Plant>{{cite web | vauthors = Schneider D |title=Innate Immunity&nbsp;– Lecture 4: Plant immune responses| publisher = Stanford University Department of Microbiology and Immunology |url=https://web.stanford.edu/class/mi104/Plant%20immunity.pdf | access-date = 1 January 2007}}</ref> Individual plant cells respond to molecules associated with pathogens known as [[pathogen-associated molecular patterns]] or PAMPs.<ref>{{cite journal | vauthors = Jones JD, Dangl JL | title = The plant immune system | journal = Nature | volume = 444 | issue = 7117 | pages = 323–29 | date = Nov 2006 | pmid = 17108957 | doi = 10.1038/nature05286 | bibcode = 2006Natur.444..323J | doi-access = free }}</ref> When a part of a plant becomes infected, the plant produces a localized [[hypersensitive response]], whereby cells at the site of infection undergo rapid [[apoptosis]] to prevent the spread of the disease to other parts of the plant. [[Systemic acquired resistance]] is a type of defensive response used by plants that renders the entire plant resistant to a particular infectious agent.<ref name= Plant /> [[RNA interference|RNA silencing]] mechanisms are particularly important in this 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 = 356–63 | date = Sep 2004 | pmid = 15372043 | doi = 10.1038/nature02874 | bibcode = 2004Natur.431..356B | s2cid = 4421274 }}</ref>