Editing Immune system (section)

== Layered defense ==
The immune system protects its host from [[infection]] with layered defenses of increasing specificity. Physical barriers prevent pathogens such as [[bacteria]] and [[virus]]es from entering the organism.{{sfn | Sompayrac | 2019 | p=1}} If a pathogen breaches these barriers, the [[innate immune system]] provides an immediate, but non-specific response. Innate immune systems are found in all [[animal]]s.<ref name=Litman>{{cite journal | vauthors = Litman GW, Cannon JP, Dishaw LJ | title = Reconstructing immune phylogeny: new perspectives | journal = Nature Reviews. Immunology | volume = 5 | issue = 11 | pages = 866–79 | date = Nov 2005 | pmid = 16261174 | pmc = 3683834 | doi = 10.1038/nri1712 }}</ref> If pathogens successfully evade the innate response, vertebrates possess a second layer of protection, the [[adaptive immune system]], which is activated by the innate response.{{sfn | Sompayrac | 2019 | p=4}} Here, the immune system adapts its response during an infection to improve its recognition of the pathogen. This improved response is then retained after the pathogen has been eliminated, in the form of an [[immunological memory]], and allows the adaptive immune system to mount faster and stronger attacks each time this pathogen is encountered.<ref name="Restifo_2013">{{cite journal | vauthors = Restifo NP, Gattinoni L | title = Lineage relationship of effector and memory T cells | journal = Current Opinion in Immunology | volume = 25 | issue = 5 | pages = 556–63 | date = October 2013 | pmid = 24148236 | doi = 10.1016/j.coi.2013.09.003 | pmc=3858177}}</ref><ref name="Kurosaki_2015">{{cite journal | vauthors = Kurosaki T, Kometani K, Ise W | title = Memory B cells | journal = Nature Reviews. Immunology | volume = 15 | issue = 3 | pages = 149–59 | date = March 2015 | pmid = 25677494 | doi = 10.1038/nri3802 | s2cid = 20825732 }}</ref>

{| class="wikitable center" style="margin: 1em auto;"
|+ '''Components of the immune system'''
! scope="col" style="background:#ccccff;" | [[#Innate immune system|Innate immune system]]
! scope="col" style="background:#ccccff;" | [[#Adaptive immune system|Adaptive immune system]]
|-
| Response is non-specific || Pathogen and [[antigen]] specific response
|-
| Exposure leads to immediate maximal response|| Lag time between exposure and maximal response
|-
| [[Cell-mediated immunity|Cell-mediated]] and [[Humoral immune response|humoral]] components|| [[Cell-mediated immunity|Cell-mediated]] and [[Humoral immune response|humoral]] components
|-
| No immunological memory || Exposure leads to immunological memory
|-
| Found in nearly all forms of life||Found only in [[Gnathostomata|jawed vertebrates]]
|}

Both innate and adaptive immunity depend on the ability of the immune system to distinguish between self and non-self [[molecule]]s. In immunology, ''self'' molecules are components of an organism's body that can be distinguished from foreign substances by the immune system.{{sfn | Sompayrac | 2019 | p=11}} Conversely, ''non-self'' molecules are those recognized as foreign molecules. One class of non-self molecules are called antigens (originally named for being ''anti''body ''gen''erators) and are defined as substances that bind to specific [[immune receptor]]s and elicit an immune response.{{sfn | Sompayrac | 2019 | p=146}}