Editing Allergy (section)

===Hygiene hypothesis===
{{Main|Hygiene hypothesis}}
Allergic diseases are caused by inappropriate immunological responses to harmless [[antigens]] driven by a [[t helper cell|TH2]]-mediated immune response. Many bacteria and viruses elicit a [[T helper cell|TH1]]-mediated immune response, which down-regulates TH2 responses. The first proposed mechanism of action of the hygiene hypothesis was that insufficient stimulation of the TH1 arm of the immune system leads to an overactive TH2 arm, which in turn leads to allergic disease.<ref>{{cite journal | vauthors = Folkerts G, Walzl G, Openshaw PJ | title = Do common childhood infections 'teach' the immune system not to be allergic? | journal = Immunology Today | volume = 21 | issue = 3 | pages = 118–20 | date = March 2000 | pmid = 10777250 | doi = 10.1016/S0167-5699(00)01582-6 }}</ref> In other words, individuals living in too sterile an environment are not exposed to enough pathogens to keep the immune system busy. Since our bodies evolved to deal with a certain level of such pathogens, when they are not exposed to this level, the immune system will attack harmless antigens, and thus normally benign microbial objects—like pollen—will trigger an immune response.<ref>{{cite web |url=http://edwardwillett.com/2000/05/the-hygiene-hypothesis/ |title=The Hygiene Hypothesis |publisher=Edward Willett |date=30 January 2013 |access-date=30 May 2013 |url-status=live |archive-url=https://web.archive.org/web/20130430180522/http://edwardwillett.com/2000/05/the-hygiene-hypothesis/ |archive-date=30 April 2013 }}</ref>

The hygiene hypothesis was developed to explain the observation that [[hay fever]] and [[eczema]], both allergic diseases, were less common in children from larger families, which were, it is presumed, exposed to more infectious agents through their siblings, than in children from families with only one child.<ref name="z256">{{cite journal | last1=Perkin | first1=Michael R | last2=Strachan | first2=David P | title=The hygiene hypothesis for allergy – conception and evolution | journal=Frontiers in Allergy | publisher=Frontiers Media SA | volume=3 | date=2022-11-24 | issn=2673-6101 | doi=10.3389/falgy.2022.1051368 | doi-access=free | page=| pmid=36506644 | pmc=9731379 }}</ref> It is used to explain the increase in allergic diseases that have been seen since industrialization, and the higher incidence of allergic diseases in more developed countries.<ref name="e520">{{cite journal | last=Bonis | first=Peter A. L. | title=Putting the Puzzle Together: Epidemiological and Clinical Clues in the Etiology of Eosinophilic Esophagitis | journal=Immunology and Allergy Clinics of North America | publisher=Elsevier BV | volume=29 | issue=1 | year=2009 | issn=0889-8561 | doi=10.1016/j.iac.2008.09.005 | pages=41–52| pmid=19141340 }}</ref> The hygiene hypothesis has now expanded to include exposure to symbiotic bacteria and parasites as important modulators of immune system development, along with infectious agents.<ref name="g732">{{cite journal | last1=Stiemsma | first1=Leah | last2=Reynolds | first2=Lisa | last3=Turvey | first3=Stuart | last4=Finlay | first4=Brett | title=The hygiene hypothesis: current perspectives and future therapies | journal=ImmunoTargets and Therapy | publisher=Informa UK Limited | year=2015 | volume=4 | pages=143–157 | issn=2253-1556 | doi=10.2147/itt.s61528 | doi-access=free | pmid=27471720 | pmc=4918254 }}</ref>

Epidemiological data support the hygiene hypothesis. Studies have shown that various immunological and autoimmune diseases are much less common in the developing world than the industrialized world, and that immigrants to the industrialized world from the developing world increasingly develop immunological disorders in relation to the length of time since arrival in the industrialized world.<ref name="pmid12910582"/> Longitudinal studies in the third world demonstrate an increase in immunological disorders as a country grows more affluent and, it is presumed, cleaner.<ref name="pmid17326711"/> The use of antibiotics in the first year of life has been linked to asthma and other allergic diseases.<ref name="Antibiotic exposure during infancy"/> The use of antibacterial cleaning products has also been associated with higher incidence of asthma, as has birth by [[caesarean section]] rather than vaginal birth.<ref name="A meta-analysis of the association between Caesarean section and childhood asthma"/><ref name="The use of household cleaning sprays and adult asthma: an international longitudinal study"/>