The hygiene hypothesis was first developed in the 1980s as an attempt to explain the increase in allergic disorders in the developed world. It postulated that the advent of sanitation and hyper vigilant hygiene practices led to reduced exposure of childhood illnesses, microorganisms and parasites. As a result, the immune system was not able to develop properly leading to the development of allergies and autoimmune disorders. This idea explains why the incidence of allergies and autoimmune disorders are so much higher in developed countries which have access to better hygiene practices, clean water and public sanitation, among other things.
Allergic disorders arise from an imbalance of the Th1/Th2 immune response with the Th2 response being dominant. It was thought that reduced contact with pathogens led to an underdevelopment of the Th1 response. However, scientists saw a problem with this theory, because there was also an increase in autoimmune disorders such as IBD, multiple sclerosis, and type 1 diabetes, which are Th1 mediated responses. This brought about the idea that the problem was not with the Th1/Th2 balance but rather with the Tregulatory cells.
The Old Friends hypothesis is a revision of the hygiene theory that takes the Treg response into account in its explanation of the problem. It states that rather than the immune system not developing properly, it has evolved in a way that requires commensal microorganisms to function. These microorganisms are treated as “friends” because they are relatively harmless or because having an immune response to them would cause more harm than benefit. Instead of attacking the microbe or parasite, the immune system goes into immunoregulatory mode increasing the maturation of both dendritic cells and Treg cells to suppress the immune response to the invader. The regulation of the “old friends” also leads to a suppression of the immune system that would cause damage to self-cells. Some studies have been performed to show that probiotics can induce Treg giving more weight to the idea that commensal bacteria are needed for proper immune function.
Citations:
Rook G. A., Brunet L. R. (2005). Microbes, immunoregulation, and the gut. Gut 54, 317–320. doi: 10.1136/gut.2004.053785. [PMC free article] [PubMed] [Cross Ref]
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1774411/
Sachs, Jessica Snyder. Good Germs, Bad Germs: Health and Survival in a Bacterial World. New York: Hill and Wang, 2008. Print.
I wonder if we just think we have more problems with the immune system now because in the past these people would not have survived childhood. As far as looking into our race's future health goes, for those disorders with genetic causes, our modern day medicine now allows those disorders to be passed down to future generations because there are more survivors living to child bearing age. Then again, we are having less offspring and can now have genetic testing done to prevent passing down some disorders.
ReplyDeleteI think you raise an interesting point about the children not surviving long enough for the problems to become apparent. Have these disorders become apparent because modern medicine allows patients to live long enough for symptoms to develop? I think this would be difficult to research or study due to a lack of accurate equipment or records in the past.
ReplyDeleteI find the immunomodulatory function of the gut microflora is fascinating, and wonder if there is a connection in lifestyle beyond a hygiene effect. What are your thoughts on diet affecting the species of bacteria that colonize the intestines? With so many different species present, is it probable to suggest that a high fat, high sugar diet could change the composition of the flora, resulting in pathological signalling with various T cells?
ReplyDeleteExercise is another healthy behavior that is seen to be beneficial in preventing cancer. I wonder if the intestines, essentially that alimentary canal which is exposed to the outside world, would be better protected from IBD with daily exercise and what the potential mechanism could be. Attenuation of hormonal influence?
Interestingly enough, I just attended a seminar this morning concerning the gut microflora and IBD. The hypothesis that the speaker put forth was that the change in diet, hygiene and increased stress overall causes a dysbiosis within the gut. This allows for bacteria that may have been symbiotic to become pathogenic. He looked particularly at how a beneficial bacteria and a pathogenic bacteria affect IBD. His finding were very interesting. The papers are below.
ReplyDeletePersonally, like StevenH495 above, I think that our westernized diet has a lot to do with the increase in inflammatory bowel disease, among other things. If you consider how hard our bodies must work to break down the food we eat and how long it can be retained within our body the idea of all those extra preservatives and additives is horrifying. Those are not natural products and could easily be seen by the immune system as antigens. Perhaps this is leading to the increase in food allergies being seen by clinicians?
Curr Opin Immunol. 2011 Aug;23(4):473-80. Epub 2011 Aug 17.Pathobionts of the gastrointestinal microbiota and inflammatory disease.Chow J, Tang H, Mazmanian SK.
Division of Biology, California Institute of Technology, Pasadena, CA 91125, USA.
http://www.ncbi.nlm.nih.gov/pubmed/21856139
Has the microbiota played a critical role in the evolution of the adaptive immune system?
Lee YK, Mazmanian SK. Science. 2010 Dec 24;330(6012):1768-73. Review.
PMID:21205662[PubMed - indexed for MEDLINE]
http://www.ncbi.nlm.nih.gov/pubmed/21205662
Host-bacterial symbiosis in health and disease.
Chow J, Lee SM, Shen Y, Khosravi A, Mazmanian SK.Adv Immunol. 2010;107:243-74. Review.
PMID:21034976 [PubMed - indexed for MEDLINE]
http://www.ncbi.nlm.nih.gov/pubmed/21034976