Editing Immune system (section)

== Surface barriers ==
Several barriers protect organisms from infection, including mechanical, chemical, and biological barriers. The waxy [[plant cuticle|cuticle]] of most leaves, the [[exoskeleton]] of insects, the [[eggshell|shells]] and membranes of externally deposited eggs, and [[skin]] are examples of mechanical barriers that are the first line of defense against infection.{{sfn|Alberts|Johnson|Lewis|Raff|2002|loc= sec. [https://www.ncbi.nlm.nih.gov/books/NBK26833/#A4639 "Pathogens Cross Protective Barriers to Colonize the Host"]}} Organisms cannot be completely sealed from their environments, so systems act to protect body openings such as the [[lung]]s, [[intestine]]s, and the [[genitourinary system|genitourinary tract]]. In the lungs, coughing and sneezing mechanically eject pathogens and other [[irritation|irritants]] from the [[respiratory tract]]. The flushing action of [[tears]] and [[urine]] also mechanically expels pathogens, while [[mucus]] secreted by the respiratory and [[gastrointestinal tract]] serves to trap and entangle [[microorganism]]s.<ref>{{cite journal | vauthors = Boyton RJ, Openshaw PJ | title = Pulmonary defences to acute respiratory infection | journal = British Medical Bulletin | volume = 61 | issue = 1 | pages = 1–12 | year = 2002 | pmid = 11997295 | doi = 10.1093/bmb/61.1.1 | doi-access = free }}</ref>

Chemical barriers also protect against infection. The skin and respiratory tract secrete [[antimicrobial peptides]] such as the β-[[defensin]]s.<ref>{{cite book | vauthors = Agerberth B, Gudmundsson GH | title = Antimicrobial Peptides and Human Disease | chapter = Host antimicrobial defence peptides in human disease | volume = 306 | pages = 67–90 | year = 2006 | pmid = 16909918 | doi = 10.1007/3-540-29916-5_3 | isbn = 978-3-540-29915-8 | series = Current Topics in Microbiology and Immunology }}</ref> [[Enzyme]]s such as [[lysozyme]] and [[phospholipase A2]] in [[saliva]], tears, and [[breast milk]] are also [[antiseptic|antibacterials]].<ref>{{cite journal|vauthors=Moreau JM, Girgis DO, Hume EB, Dajcs JJ, Austin MS, O'Callaghan RJ |title=Phospholipase A(2) in rabbit tears: a host defense against Staphylococcus aureus |journal=Investigative Ophthalmology & Visual Science |volume=42 |issue=10 |pages=2347–54 |date=Sep 2001 |pmid=11527949 |url=http://iovs.arvojournals.org/article.aspx?articleid=2200058}}</ref><ref>{{cite journal | vauthors = Hankiewicz J, Swierczek E | title = Lysozyme in human body fluids | journal = Clinica Chimica Acta; International Journal of Clinical Chemistry | volume = 57 | issue = 3 | pages = 205–09 | date = Dec 1974 | pmid = 4434640 | doi = 10.1016/0009-8981(74)90398-2 }}</ref> [[Vagina]]l secretions serve as a chemical barrier following [[menarche]], when they become slightly [[acid]]ic, while [[semen]] contains defensins and [[zinc]] to kill pathogens.<ref>{{cite journal | vauthors = Fair WR, Couch J, Wehner N | title = Prostatic antibacterial factor. Identity and significance | journal = Urology | volume = 7 | issue = 2 | pages = 169–77 | date = Feb 1976 | pmid = 54972 | doi = 10.1016/0090-4295(76)90305-8 }}</ref><ref>{{cite journal | vauthors = Yenugu S, Hamil KG, Birse CE, Ruben SM, French FS, Hall SH | title = Antibacterial properties of the sperm-binding proteins and peptides of human epididymis 2 (HE2) family; salt sensitivity, structural dependence and their interaction with outer and cytoplasmic membranes of Escherichia coli | journal = The Biochemical Journal | volume = 372 | issue = Pt 2 | pages = 473–83 | date = Jun 2003 | pmid = 12628001 | pmc = 1223422 | doi = 10.1042/BJ20030225 }}</ref> In the [[stomach]], [[gastric acid]] serves as a chemical defense against ingested pathogens.<ref>{{cite journal|vauthors=Smith JL|year=2003|title=The role of gastric acid in preventing foodborne disease and how bacteria overcome acid conditions|journal=J Food Prot|volume=66|issue=7|pages=1292–1303|pmid=12870767|doi=10.4315/0362-028X-66.7.1292|doi-access=free}}</ref>

Within the genitourinary and gastrointestinal tracts, [[commensalism|commensal]] [[gut flora|flora]] serve as biological barriers by competing with pathogenic bacteria for food and space and, in some cases, changing the conditions in their environment, such as [[pH]] or available iron. As a result, the probability that pathogens will reach sufficient numbers to cause illness is reduced.<ref>{{cite journal | vauthors = [[Sherwood Gorbach|Gorbach SL]] | title = Lactic acid bacteria and human health | journal = Annals of Medicine | volume = 22 | issue = 1 | pages = 37–41 | date = Feb 1990 | pmid = 2109988 | doi = 10.3109/07853899009147239 }}</ref>