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Cleanliness Equals More Sickliness? March 4, 2008

Posted by tkcollier in Enviroment.
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Immune Systems Increasingly On Attack – washingtonpost.com
Our obsession with germs and cleanliness is not exposing children the ability to develop resistance and anti-bodies.  Some studies now indicate that more than half of the U.S. population has at least one allergy.

The cause remains the focus of intense debate and study, but some researchers suspect the concurrent trends all may have a common explanation rooted in aspects of modern living — including the “hygiene hypothesis” that blames growing up in increasingly sterile homes, changes in diet, air pollution, and possibly even obesity and increasingly sedentary lifestyles.

One reason that many researchers suspect something about modern living is to blame is that the increases show up largely in highly developed countries in Europe, North America and elsewhere, and have only started to rise in other countries as they have become more developed.

“It’s striking,” said William Cookson of the Imperial College in London.

The leading theory to explain the phenomenon holds that as modern medicine beats back bacterial, viral and parasitic diseases that have long plagued humanity, immune systems may fail to learn how to differentiate between real threats and benign invaders, such as ragweed pollen or food. Or perhaps because they are not busy fighting real threats, they overreact or even turn on the body’s own tissues.

“Our immune systems are much less busy,” said Jean-Francois Bach of the French Academy of Sciences, “and so have much more strong responses to much weaker stimuli, triggering allergies and autoimmune diseases.”

Several lines of evidence support the theory. Children raised with pets or older siblings are less likely to develop allergies, possibly because they are exposed to more microbes. But perhaps the strongest evidence comes from studies comparing thousands of people who grew up on farms in Europe to those who lived in less rural settings. Those reared on farms were one-tenth as likely to develop diseases such as asthma and hay fever.

“The data are very strong,” said Erika von Mutius of the Ludwig-Maximilians University in Munich. “If kids have all sorts of exposures on the farm by being in the stables a lot, close to the animals and the grasses, and drinking cow’s milk from their own farm, that seems to confer protection.”

The theory has also gained support from a variety of animal studies. One, for example, found that rats bred in a sterile laboratory had far more sensitive immune systems than those reared in the wild, where they were exposed to infections, microorganisms and parasites.

“It’s sort of a smoking gun of the hygiene hypothesis,” said William Parker of Duke University.

Researchers believe the lack of exposure to potential threats early in life leaves the immune system with fewer command-and-control cells known as regulatory T cells, making the system more likely to overreact or run wild.

“If you live in a very clean society, you’re not going to have a lot of regulatory T cells,” Parker said.

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