Editing Allergy (section)

===Other environmental factors===
Allergic diseases are more common in industrialized countries than in countries that are more traditional or agricultural, and there is a higher rate of allergic disease in urban populations versus rural populations, although these differences are becoming less defined.<ref name=cooper04>{{cite journal | vauthors = Cooper PJ | title = Intestinal worms and human allergy | journal = Parasite Immunology | volume = 26 | issue = 11–12 | pages = 455–67 | year = 2004 | pmid = 15771681 | doi = 10.1111/j.0141-9838.2004.00728.x | s2cid = 23348293 }}</ref> Historically, the trees planted in urban areas were predominantly male to prevent litter from seeds and fruits, but the high ratio of male trees causes high pollen counts, a phenomenon that horticulturist Tom Ogren has called "[[botanical sexism]]".<ref>{{cite web | vauthors = Ogren TL |title=Botanical Sexism Cultivates Home-Grown Allergies |url=https://blogs.scientificamerican.com/guest-blog/botanical-sexism-cultivates-home-grown-allergies/ |website=[[Scientific American]] |date=29 April 2015 |access-date=18 January 2020}}</ref>

Alterations in exposure to [[microorganism]]s is another plausible explanation, at present, for the increase in [[Atopy|atopic allergy]].<ref name= Janeway/> Endotoxin exposure reduces release of inflammatory [[cytokine]]s such as [[tumor necrosis factor alpha|TNF-α]], [[interferon-gamma|IFNγ]], [[interleukin-10]], and [[interleukin-12]] from white blood cells ([[leukocytes]]) that circulate in the blood.<ref name="pmid12239255"/> Certain microbe-sensing [[protein]]s, known as [[Toll-like receptor]]s, found on the surface of cells in the body are also thought to be involved in these processes.<ref name="Epidemiological and immunological evidence for the hygiene hypothesis"/>

[[Parasitic worm]]s and similar parasites are present in untreated drinking water in developing countries, and were present in the water of developed countries until the routine [[Water chlorination|chlorination]] and purification of drinking water supplies.<ref name="Parasitic food-borne and water-borne zoonoses"/> Recent research has shown that some common parasites, such as intestinal worms (e.g., [[hookworm]]s), secrete chemicals into the gut wall (and, hence, the bloodstream) that [[immunosuppressant|suppress]] the immune system and prevent the body from attacking the parasite.<ref name="Worms and allergy"/> This gives rise to a new slant on the hygiene hypothesis theory—that [[co-evolution]] of humans and parasites has led to an immune system that functions correctly only in the presence of the parasites. Without them, the immune system becomes unbalanced and oversensitive.<ref name=Yazdanbakhsh02>{{cite journal | vauthors = [[Maria Yazdanbakhsh|Yazdanbakhsh M]], Kremsner PG, van Ree R | title = Allergy, parasites, and the hygiene hypothesis | journal = Science | volume = 296 | issue = 5567 | pages = 490–94 | date = April 2002 | pmid = 11964470 | doi = 10.1126/science.296.5567.490 | bibcode = 2002Sci...296..490Y | citeseerx = 10.1.1.570.9502 }}</ref>

In particular, research suggests that allergies may coincide with the delayed establishment of [[gut flora]] in [[infant]]s.<ref name="pmid17382394" /> However, the research to support this theory is conflicting, with some studies performed in China and [[Ethiopia]] showing an increase in allergy in people infected with intestinal worms.<ref name="cooper04" /> Clinical trials have been initiated to test the effectiveness of certain worms in treating some allergies.<ref name="falcone05">{{cite journal | vauthors = Falcone FH, Pritchard DI | title = Parasite role reversal: worms on trial | journal = Trends in Parasitology | volume = 21 | issue = 4 | pages = 157–60 | date = April 2005 | pmid = 15780835 | doi = 10.1016/j.pt.2005.02.002 }}</ref> It may be that the term 'parasite' could turn out to be inappropriate, and in fact a hitherto unsuspected [[Mutualism (biology)|symbiosis]] is at work.<ref name="falcone05" /> For more information on this topic, see [[Helminthic therapy]].