This feature article by Megan Scudellari in Proceedings of the National Academy of Sciences is a well-researched review on the rise of allergy and autoimmune diseases, from which she concludes “it’s obviously due to much more than rampant cleanliness”. She reflects on whether it is possible to abandon the term hygiene hypothesis and thereby detach the concept from its association with “hygiene” as the public understands it.
Megan traces the issue from the 1989 “hygiene hypothesis” proposal – the idea that the surge in autoimmune and allergic disorders and simultaneous sharp decline in mumps, measles, tuberculosis, and other infectious diseases were connected in some way. Studies soon showed that this hypothesis did not hold up - infections like measles and many respiratory diseases proved not to be protective against allergic disease.
In 2003, Graham Rook proposed a revised explanation - that early exposure to “old friends” microorganisms present throughout human evolution train the immune system to react appropriately. Rook likens the immune system to a computer programme: It has software, but needs data—in the form of exposure to a diverse set of microbes—to train it to identify threats appropriately. It’s not about just learning what to attack, but learning what to tolerate. The problem comes when our immune system meets an allergen like pollen or peanuts and doesn’t know that is harmless.
Rook argues that hygiene in its broad sense - improvements in sanitation, food, and water in the late 20th century - were involved in reducing our exposure to old friends microbes, but simultaneous changes in other factors most likely had an even larger influence especially in early life. Caesarean sections have been linked to increased risk of allergy and asthma, whilst owning a pet or growing up on a farm is protective. Antibiotic use (which kills off both good and bad microbes) has been linked to asthma, cow’s milk allergy, IBD, and eczema.
Looking to the future, Meghan concludes that although we have now moved beyond the “hygiene” hypothesis, there are very few credible studies testing interventions to reregulate the immune system. She quotes Wills-Karp as saying “Researchers hope that, at some point, they will identify which regulatory pathways train the immune system. By finding common pathways, we could adopt drugs or probiotics to activate them to condition the immune system properly in early life.”
This may seem exiting - but meanwhile the media will continue to persuade us that we have become too clean for our own good - and we need to stop washing our hands – despite the fact that this is probably the most cost effective means to reduce the global burden of infectious disease - and key to tackling the global problem of antibiotic resistance? The article is open access at http://www.pnas.org/content/114/7/1433.full.pdf
