What is Adaptive immune system?
[[File:SEM Lymphocyte.jpg|thumb|right|200px|A [[scanning electron microscope]] (SEM) image of a single human [[lymphocyte]].]] The '''adaptive immune system''' is made of specialized [[cell]]s and [[process]]es which kill [[pathogen]]s or prevent their attack. The adaptive immune system is switched on by the [[evolution]]arily older [[innate immune system]].<ref name=Evol/> This older system is non-specific, whereas the adaptive system is tailored to specific targets. Whereas the innate immune system is found in all [[metazoa]], the adaptive system is only found in [[vertebrate]]s. It is thought to have arisen in the first [[Gnathostomata|jawed vertebrates]].<ref name=Evol>Janeway C.A. 2001. Evolution of the immune system. In ''Immunobiology'' ed Janeway et al. 5th ed, 597{{ndash}}607. New York: Garland Science. {{ISBN|978-0-8153-4101-7}}</ref><ref>{{Citation|last1=Hirano|first1=Masayuki|title=The Evolution of Adaptive Immunity in Vertebrates |date=2011-01-01 |url=http://www.sciencedirect.com/science/article/pii/B9780123876645000042|series=Advances in Immunology|volume=109|pages=125–157|editor-last=Alt|editor-first=Frederick W.|publisher=Academic Press|doi=10.1016/b978-0-12-387664-5.00004-2|pmid=21569914|access-date=2019-12-03|last2=Das|first2=Sabyasachi|last3=Guo|first3=Peng|last4=Cooper|first4=Max D.|isbn=9780123876645}}</ref> The adaptive immune response gives the vertebrate [[immune system]] the ability to recognize and remember specific pathogens. The system mounts stronger attacks each time a particular pathogen is encountered. It is ''adaptive'' immunity because the body's immune system prepares itself for future challenges. ==Layered defence== The immune system protects organisms from [[infection]] with layered defences. In simple terms, physical barriers prevent pathogens such as [[bacteria]] and [[virus]]es from entering the organism. If a pathogen breaches these barriers, the [[innate immune system]] provides an immediate, but non-specific response. Innate immune systems are found in all [[plant]