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Let them eat dirt!

LONDON, UNITED KINGDOM. Allergic diseases are on the rise. Some 300 million people worldwide now suffer from asthma and it is estimated that another 100 million will be added to their ranks over the next 20 years. Allergic rhinitis (including hay fever and reactions to dust mites and cat dander) affects 40% of all American children and peanut allergies doubled between 1997 and 2002.

Clearly something has gone awry – the question is what? A few years ago it was thought that the widespread use of vaccination was to blame (hygiene hypothesis) because it prevented childhood infections that normally would have helped teach the immune system how to react appropriately to foreign invaders. This hypothesis has now been pretty well discredited with the discovery, by Japanese scientists, that the allergic response is not due to an imbalance between T1 and T2 helper cells as postulated by the hygiene hypothesis, but rather to reduced effectiveness of a newly discovered class of T-cells called regulatory T- cells. These cells hold back an excessive response by T helper cells and thus prevent the allergic response. There is now increasing evidence that exposure to microbes contained in dirt, untreated water, and farm animals is essential for the proper development of regulatory T-cells.

Studies in Australia, Europe and the US have consistently shown that children growing up in a rural environment have a far smaller risk of developing allergies than do children reared in an urban environment. Work is now underway to find out exactly what it is in the rural environment that primes children to avoid allergies.
Hamilton, Garry. Filthy friends. New Scientist, April 16, 2005, pp. 34-39

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