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The hygiene hypothesis: the good, the bad, and the evil - a low-down on dirt

, : The hygiene hypothesis: the good, the bad, and the evil - a low-down on dirt. New Zealand Journal of Medical Laboratory Science 57(3): 111-115, November

Asthma and atopic diseases are on the rise throughout the world, especially in developed countries, where children from affluent households live in cleaner, relatively aseptic environments, as part of the so-called Western style of life. Many hypotheses have been proposed to explain the increases in atopic diseases and in particular asthma. The most widely discussed and the most controversial - is the so-called "hygiene hypothesis". The hygiene hypothesis states that the western lifestyle has caused a decrease in the incidence of infections early in life, but these infections may have a protective effect on the subsequent development of atopic diseases. This hypothesis proposes that infections early in life help to mature the immune system to a Th1 biased state and not a Th2 state, the latter predisposing the child to atopic disease. One substance proposed to shift this immune system balance to the Th1 state, hence altering the outcome of atopy, is bacterial endotoxin. While many epidemiological studies support the hygiene hypothesis in the development of atopic diseases, including asthma, the evidence remains inconclusive. It is possible that other environmental exposures in early childhood also alter the predisposition towards asthma. Both the affirmative and negative views on the hygiene hypothesis are reviewed in this paper.


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