Editing Appendix (anatomy) (section)

==Functions==

===Maintaining gut flora===
[[File:Appendix function diagram.svg|left|thumb|A possible function of the human appendix is a "safe house" for beneficial bacteria in the recovery from [[diarrhea]]]]

Although it has been long accepted that the immune tissue surrounding the appendix and elsewhere in the gut—called [[gut-associated lymphoid tissue]]—carries out several important functions, explanations were lacking for the distinctive shape of the appendix and its apparent lack of specific importance and function as judged by an absence of side effects following [[appendectomy|its removal]].<ref name="PBD4thEd">{{Cite book |author1=Kumar, Vinay |author2=Robbins, Stanley L. |author3=Cotran, Ramzi S. |title=Robbins' pathologic basis of disease |publisher=Saunders |location=Philadelphia |year=1989 |pages=[https://archive.org/details/robbinspathologi00robb/page/902 902–3] |isbn=978-0-7216-2302-3 |edition=4th |url-access=registration |url=https://archive.org/details/robbinspathologi00robb/page/902 }}</ref> Therefore, the notion that the appendix is only [[vestigiality|vestigial]] became widely held.

William Parker, Randy Bollinger, and colleagues at [[Duke University]] proposed in 2007 that the appendix serves as a haven for useful [[bacteria]] when illness flushes the bacteria from the rest of the intestines.<ref name=health>{{cite news |agency=[[Associated Press]] |date=5 October 2007 |title=Scientists may have found appendix's purpose |url=http://www.nbcnews.com/id/21153898/ |url-status=dead |work=[[NBC News]] |archive-url=https://web.archive.org/web/20200204135004/http://www.nbcnews.com/id/21153898 |archive-date=4 February 2020 |access-date=24 August 2019}}</ref><ref name="JTBpaper">{{cite journal | vauthors = Randal Bollinger R, Barbas AS, Bush EL, Lin SS, Parker W | title = Biofilms in the large bowel suggest an apparent function of the human vermiform appendix | journal = Journal of Theoretical Biology | volume = 249 | issue = 4 | pages = 826–31 | date = December 2007 | pmid = 17936308 | doi = 10.1016/j.jtbi.2007.08.032 | bibcode = 2007JThBi.249..826R }}</ref> This proposition is based on an understanding that emerged by the early 2000s of how the immune system supports the growth of beneficial [[gut flora|intestinal bacteria]],<ref>{{cite journal | vauthors = Sonnenburg JL, Angenent LT, Gordon JI | title = Getting a grip on things: how do communities of bacterial symbionts become established in our intestine? | journal = [[Nature Immunology]] | volume = 5 | issue = 6 | pages = 569–73 | date = June 2004 | pmid = 15164016 | doi = 10.1038/ni1079 | s2cid = 25672527 }}</ref><ref>{{cite journal |author1=Everett M.L. |author2=Palestrant D. |author3=Miller S.E. |author4=Bollinger R.R. |author5=Parker W. |title=Immune exclusion and immune inclusion: a new model of host-bacterial interactions in the gut |journal=Clinical and Applied Immunology Reviews |volume=4 |pages=321–32 |year=2004|doi=10.1016/j.cair.2004.03.001|issue=5}}</ref> in combination with many well-known features of the appendix, including its architecture, its location just below the normal one-way flow of food and germs in the large intestine, and its association with copious amounts of immune tissue. 

Research performed at [[Winthrop–University Hospital]] showed that individuals without an appendix were four times as likely to have a recurrence of [[Clostridioides difficile infection|''Clostridioides difficile'' colitis]].<ref>{{cite web |url=https://blogs.scientificamerican.com/guest-blog/2012/01/02/your-appendix-could-save-your-life/ |title=Your Appendix Could Save Your Life |last=Dunn |first=Rob |date=January 2, 2012 |publisher=[[Scientific American]] |access-date=22 December 2016 |url-status=live |archive-url=https://web.archive.org/web/20201111234503/https://blogs.scientificamerican.com/guest-blog/your-appendix-could-save-your-life/ |archive-date=11 November 2020}}</ref> The appendix, therefore, may act as a "safe house" for beneficial bacteria.<ref name="health" /> This reservoir of bacteria could then serve to repopulate the [[gut flora]] in the [[human digestive system|digestive system]] following a bout of [[dysentery]] or [[cholera]] or to boost it following a milder gastrointestinal illness.<ref name="JTBpaper"/>

===Immune and lymphatic systems===
The appendix has been identified as an important component of [[mammal]]ian [[mucosal immunology|mucosal immune function]], particularly [[B cell]]-mediated immune responses and [[thymus|extrathymically]] derived [[T cell]]s. This structure helps in the proper movement and removal of waste matter in the digestive system, contains lymphatic vessels that regulate pathogens, and lastly, might even produce early defences that prevent deadly diseases. Additionally, it is thought that this may provide more immune defences from invading pathogens and getting the lymphatic system's B and T cells to fight the viruses and bacteria that infect that portion of the bowel and training them so that immune responses are targeted and more able to reliably and less dangerously fight off pathogens.<ref>{{cite journal | vauthors = Zahid A | title = The vermiform appendix: not a useless organ | journal = Journal of the College of Physicians and Surgeons--Pakistan | volume = 14 | issue = 4 | pages = 256–8 | date = April 2004 | pmid = 15228837 }}</ref> In addition, there are different immune cells called [[innate lymphoid cell]]s that function in the gut to help the appendix maintain digestive health.<ref>{{cite journal|display-authors=6|vauthors=Rankin LC, Girard-Madoux MJ, Seillet C, Mielke LA, Kerdiles Y, Fenis A, Wieduwild E, Putoczki T, Mondot S, Lantz O, Demon D, Papenfuss AT, Smyth GK, Lamkanfi M, Carotta S, Renauld JC, Shi W, Carpentier S, Soos T, Arendt C, Ugolini S, Huntington ND, Belz GT, Vivier E|date=February 2016|title=Complementarity and redundancy of IL-22-producing innate lymphoid cells|journal=[[Nature Immunology]]|volume=17|issue=2|pages=179–86|doi=10.1038/ni.3332|pmc=4720992|pmid=26595889}}</ref> 

Research also shows a positive correlation between the existence of the appendix and the concentration of cecal lymphoid tissue, which supports the suggestion that not only does the appendix evolve as a complex with the cecum but also has major immune benefits.<ref>{{cite journal | vauthors = Smith H, Parker W, Kotze S, Laurin M| title = Morphological evolution of the mammalian cecum and cecal appendix | journal =  Comptes Rendus Palevol| volume = 16 | issue = 1 | pages = 39–57 | date = September 2016 |doi=10.1016/j.crpv.2016.06.001| url = https://zenodo.org/record/894682 | doi-access = free }}</ref>