Hygiene hypothesis 407814 224565042 2008-07-09T13:03:16Z PieCam 2139616 [[WP:UNDO|Undid]] revision 224088510 by [[Special:Contributions/FQ1513|FQ1513]] ([[User talk:FQ1513|talk]]) Link does not work. Fixing it points to a site mostly advocating "worm therapy". In [[medicine]], the '''hygiene hypothesis''' states that a lack of early childhood exposure to infectious agents, [[symbiotic]] microorganisms (e.g. [[gut flora]]), and parasites increases susceptibility to [[allergy|allergic]] diseases by modulating immune system development.<ref>Strachan DP. Family size, infection and atopy: the first decade of the "hygiene hypothesis". Thorax 55 Suppl 1:S2-10.: S2-10, 2000.</ref> ==History== First proposed by David P. Strachan in an article published in the ''[[British Medical Journal]]'' (now the ''BMJ''), in 1989,<ref>Strachan DP. Hay fever, hygiene, and household size. BMJ 299: 1259-1260, 1989.</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 presumably exposed to more infectious agents through their siblings, than in children from families with only one child. The hygiene hypothesis has been extensively investigated by [[immunology|immunologists]] and [[epidemiology|epidemiologists]] and has become an important theoretical framework for the study of allergic disorders. It is used to explain the increase in allergic diseases that has been seen since industrialization, and the higher incidence of allergic diseases in more developed countries. 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 (see [[#Current theories|Current theories]] below). ==Mechanism of action== 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 [[TH1]]-mediated immune response, which down-regulates TH2 responses. The first proposed mechanism of action of the hygiene hypothesis stated that insufficient stimulation of the TH1 arm of the immune system lead to an overactive TH2 arm, which in turn led to allergic disease.<ref>Folkerts G, Walzl G, Openshaw PJ. Do common childhood infections 'teach' the immune system not to be allergic? Immunol Today 2000; 21(3):118-120. [http://www.ncbi.nlm.nih.gov/sites/entrez?cmd=Retrieve&db=PubMed&list_uids=10777250&dopt=Abstract PubMed]</ref> The first proposed mechanistic explanation for the hygiene hypothesis cannot explain the rise in incidence (similar to the rise of allergic diseases) of several TH1-mediated [[autoimmunity|autoimmune diseases]], including [[inflammatory bowel disease]] ([[IBD]]), [[multiple sclerosis]] ([[MS]]), and [[type I diabetes]]. The major proposed alternative mechanistic explanation is that the developing immune system must receive stimuli (from infectious agents, symbiotic bacteria, or parasites) in order to adequately develop [[regulatory T cells]], or it will be more susceptible to autoimmune diseases and allergic diseases, because of insufficiently repressed TH1 and TH2 responses, respectively.<ref>Guarner F, Bourdet-Sicard R, Brandtzaeg P, Gill HS, McGuirk P, van EW, Versalovic J, Weinstock JV and Rook GA. Mechanisms of disease: the hygiene hypothesis revisited. Nat Clin Pract Gastroenterol Hepatol 3: 275-284, 2006. [http://www.ncbi.nlm.nih.gov/sites/entrez?cmd=Retrieve&db=PubMed&list_uids=15878454&dopt=Abstract PubMed]</ref> ==Current theories== The hygiene hypothesis has expanded to include exposure to several varieties of microorganisms and parasites, with which humans coexisted throughout much of our evolutionary history, as necessary for balanced and regulated immune system development.<ref>{{cite journal |author=Gold DR,Wright R |title=Population disparities in asthma |journal=Annu Rev Public Health |volume=26 |pages=89–113 |year=2005 |pmid=15760282 |doi=10.1146/annurev.publhealth.26.021304.144528}}</ref> In recent times, the development of hygienic practices and effective medical care have diminished or eliminated exposure to these microorganisms and parasites during development. Examples of organisms that may be important for proper development of T regulatory cells include [[lactobacillus|lactobacilli]], various [[mycobacterium|mycobacteria]], and certain [[helminths]].<ref>{{cite journal |author=Rook GA,Brunet LR |title=Old friends for breakfast |journal=Clin Exp Allergy |volume=35 |issue=7 |pages=841–2 |year=2005 |pmid=16008666 |doi=10.1111/j.1365-2222.2005.02112.x}}</ref> Epidemiological data supports 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">{{cite journal |author=Gibson PG, Henry RL, Shah S, Powell H, Wang H |title=Migration to a western country increases asthma symptoms but not eosinophilic airway inflammation |journal=[[Pediatr. Pulmonol.]] |volume=36 |issue=3 |pages=209–15 |year=2003 |month=September |pmid=12910582 |doi=10.1002/ppul.10323 |url=http://dx.doi.org/10.1002/ppul.10323 |issn= |accessdate=2008-07-06}}</ref> Longitudinal studies in the third world demonstrate an increase in immunological disorders as a country grows more affluent and, presumably, cleaner<ref name="pmid17326711">{{cite journal |author=Addo-Yobo EO, Woodcock A, Allotey A, Baffoe-Bonnie B, Strachan D, Custovic A |title=Exercise-induced bronchospasm and atopy in Ghana: two surveys ten years apart |journal=[[PLoS Med.]] |volume=4 |issue=2 |pages=e70 |year=2007 |month=February |pmid=17326711 |pmc=1808098 |doi=10.1371/journal.pmed.0040070 |url=http://medicine.plosjournals.org/perlserv/?request=get-document&doi=10.1371/journal.pmed.0040070 |issn= |accessdate=2008-07-06}}</ref>. The use of antibiotics in the first year of life has been linked to asthma and other allergic diseases.<ref>{{cite journal |author=Marra F, Lynd L, Coombes M "et al." |title=Does antibiotic exposure during infancy lead to development of asthma?: a systematic review and metaanalysis |journal=Chest |volume=129 |issue=3 |pages=610–8 |year=2006 |pmid=16537858 |doi=10.1378/chest.129.3.610}}</ref> 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>{{cite journal |author=Thavagnanam S, Fleming J, Bromley A, Shields MD, Cardwell, CR |title=A meta-analysis of the association between Caesarean section and childhood asthma |journal=Clin. and Exper. Allergy |volume=online ahead of print |year=2007 |doi=10.1111/j.1365-2222.2007.02780.x |pages=629}}</ref><ref>{{cite journal |author=Zock JP, Plana E, Jarvis D "et al." |title=The use of household cleaning sprays and adult asthma: an international longitudinal study| journal=Am J Respir Crit Care Med |volume=176 |issue=8 |pages=735–41| year=2007 |pmid=17585104 |doi=10.1164/rccm.200612-1793OC}}</ref> However, the studies investigating these links showed only tenuous correlation between the factors described and the conditions they are theorized to cause. ==See also== *[[Hookworm#Hookworm in therapy|Hookworm in therapy]] *[[Antibacterial soap]] *[[Helminthic therapy]] ==References== {{reflist}} ===Additional references=== 1. Camateros P, Moisan J, Henault J, De SJ, Skamene E and Radzioch D. Toll-like receptors, cytokines and the immunotherapeutics of asthma. Curr Pharm Des 12: 2365-2374, 2006. ==External links== *[http://www.pubmedcentral.nih.gov/articlerender.fcgi?artid=1299202 Should auld acquaintance be forgot...] *[http://www.pubmedcentral.nih.gov/articlerender.fcgi?artid=416594 the defence of dirt] *[http://gut.bmj.com/cgi/content/full/54/3/317 Microbes, immunoregulation, and the gut] *[http://www.springerlink.com/content/0byj25luk2l8e6f8/ Mycobacteria and other environmental organisms as immunomodulators for immunoregulatory disorders ] *[http://www.nytimes.com/2007/11/27/health/27book.html?ref=science Separating Friend From Foe Among the Body’s Invaders] *[http://healthlink.mcw.edu/article/1031002421.html Hygiene Hypothesis: Are We Too "Clean" for Our Own Good?] *[http://www.boston.com/news/science/articles/2007/12/31/his_parasite_theory_stirs_a_revolution/ His Parasite Theory Stirs a Revolution] [[Category:Allergology]][[Category:Epidemiology]]