FEATURE
The Hygiene Hypothesis By Muhtashim Mian
I
n his stand-up comedy special, “You Are All Diseased” the late George Carlin attributes his ability to avoid sickness to be the result of swimming “in raw sewage” as a child. While his routine is wildly entertaining, if exaggerated, the sentiment behind it nevertheless may carry some weight.
24 | IMS MAGAZINE WINTER 2016 IMMUNOLOGY
Intuitively, it makes sense that our immune system would undergo the greatest development during childhood. Given that we are a product of both environment and genetics, it follows that an ideal environment exists which would foster the development of an optimal immune system. Several studies give us a clue about what this ideal environment may be. As per Strachan’s findings, important environmental factors in developing atopic disease seem to be family size and birth order. More specifically, a strong protective effect against atopic disease has been shown for children with older brothers rather than sisters.2 One explanation in support of the hygiene hypothesis is that older brothers are generally more frequently in contact with dirt than sisters, presumably because of playing habits.
Another explanation entirely different from this hypothesis may be that that changes in the mother after each birth have a protective effect for subsequent offspring. Indeed, some studies have shown significant decreases in maternal and cord blood IgE (the antibody responsible for allergic reactions) after each birth.3 While the maternal environment may have some effect, it alone cannot sufficiently explain other observations related to external childhood environment. For example, close contact, such as bedroom sharing, which is more common in larger families, provided protection against subsequent atopic disease.2 Similarly, it has been shown that children that attended large daycare facilities had lower chance of asthma later in life.4 Although evidence for daycare attendance is inconsistent, the data suggests that at worst, there is no protective effect,5 and at best, there is significant protection against atopy (becoming hyperallergic).6 Another interesting environmental factor seems to be the type of community a child is raised: farming children, as opposed to rural children with no agricultural
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The idea that early exposure to “germs,” or lack thereof, affects the state of the immune system in adulthood is not new. As far back as 1989, David P. Strachan noticed a steady increase of atopic or allergy-related diseases over the 20th century.1 He also noticed that children with many older siblings tended to have lower incidence of such diseases. Taken together with apparent improvements in public cleanliness and sanitation laws, Strachan formally proposed what is now known as the hygiene hypothesis. He suggested that the rise of hay fever, an atopic disease caused by allergy to pollen or dust, was a result of children not being exposed to infectious agents as often. He argued that children with older siblings were more likely to be exposed to infectious agents due to frequent
unhygienic contact. Thus, the hypothesis was predicated on two essential points: 1) microbial exposure and infections during childhood have protective effect against atopic diseases and 2) the growing trend of cleanliness prevents such infections from taking place.