Editing Gastrointestinal tract (section)

=====Immune barrier=====
The surface area of the digestive tract is estimated to be about 32 square meters, or about half a badminton court.<ref name="ReferenceA" /> With such a large exposure (more than three times larger than the [[Human skin|exposed surface of the skin]]), these immune components function to prevent pathogens from entering the blood and lymph circulatory systems.<ref>{{Cite journal|last1= Flannigan|first1=Kyle L.|last2=Geem|first2=Duke|last3=Harusato|first3=Akihito|last4=Denning|first4=Timothy L.|date=2015-07-01|title=Intestinal Antigen-Presenting Cells: Key Regulators of Immune Homeostasis and Inflammation|journal=The American Journal of Pathology|volume=185|issue=7|pages=1809–1819|doi= 10.1016/j.ajpath.2015.02.024|issn=1525-2191|pmc=4483458|pmid=25976247}}</ref> Fundamental components of this protection are provided by the [[intestinal mucosal barrier]], which is composed of physical, biochemical, and immune elements elaborated by the intestinal mucosa.<ref>{{Cite journal|last1=Sánchez de Medina|first1=Fermín|last2=Romero-Calvo|first2=Isabel|last3=Mascaraque|first3=Cristina|last4=Martínez-Augustin|first4=Olga|date=2014-12-01|title= Intestinal inflammation and mucosal barrier function|journal=Inflammatory Bowel Diseases|volume=20|issue=12|pages= 2394–2404|doi=10.1097/MIB.0000000000000204|issn=1536-4844|pmid=25222662|s2cid=11434730|doi-access=free}}</ref> Microorganisms also are kept at bay by an extensive immune system comprising the [[gut-associated lymphoid tissue]] (GALT).

There are additional factors contributing to protection from pathogen invasion. For example, low [[pH]] (ranging from 1 to 4) of the stomach is fatal for many [[microorganism]]s that enter it.<ref>{{Cite journal|last=Schubert|first=Mitchell L.|date=2014-11-01|title=Gastric secretion|journal=Current Opinion in Gastroenterology|volume=30|issue=6|pages=578–582|doi=10.1097/MOG.0000000000000125|issn=1531-7056|pmid=25211241|s2cid=8267813}}</ref> Similarly, [[mucus]] (containing [[IgA]] [[antibody|antibodies]]) neutralizes many pathogenic microorganisms.<ref>{{Cite journal|last1=Márquez|first1=Mercedes|last2=Fernández Gutiérrez Del Álamo|first2=Clotilde|last3=Girón-González|first3=José Antonio|date=2016-01-28|title=Gut epithelial barrier dysfunction in human immunodeficiency virus-hepatitis C virus coinfected patients: Influence on innate and acquired immunity|journal=World Journal of Gastroenterology|volume=22|issue=4|pages=1433–1448|doi=10.3748/wjg.v22.i4.1433|issn=2219-2840|pmc=4721978|pmid=26819512|doi-access=free}}</ref> Other factors in the GI tract contribution to immune function include [[enzyme]]s secreted in the [[saliva]] and [[bile]].