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Allergy & Asthma Advocate: Winter 2008
Hygiene Hypothesis
By Harvey L. Leo, MD, FAAAAI 
The hygiene hypothesis is a general hypothesis stating that as society changes our environmental exposures from a more rural landscape with animals, bacteria and “dirt,” to a more sterile environment, the immune system has begun to over-react to food items, environmental stimuli and viruses that were once considered benign. In very simple terms, as we get cleaner, we may see more allergic disease.
There are many layers to the hygiene hypothesis that go well beyond, “dirt is good, clean is bad.” Some research has shown that endotoxins (a byproduct of certain bacterial cell walls) can play a major role in terms of how the immature immune system develops. Larger community studies have shown that allergic diseases are more prevalent in more industrialized countries compared to ones that are considered rural.
Recently, the hygiene hypothesis has received much attention in the media, as well as in scientific literature regarding the development of allergic disease and other diseases such as diabetes, lupus and other chronic diseases.
Although the hypothesis has many supporters, careful interpretation of it is important. Families should not assume that if a person is already allergic, that they can make their environment “dirtier” to make it less allergic, or in fact reduce the chances of children becoming allergic.
Nevertheless, the hygiene hypothesis allows for researchers to understand the inner workings of the immune system at a very early stage and it should produce new strategies of how to handle the rise of allergic disease.
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