Immune Systems Increasingly On Attack

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.

While the evidence for the hygiene theory is accumulating, many say it remains far from proven.

"That theory is so full of holes that it's clearly not the whole story," said Robert Wood of the Johns Hopkins School of Medicine.

It does not explain, for example, the rise in asthma, since that disease occurs much more commonly in poor, inner-city areas where children are exposed to more cockroaches and rodents that may trigger it, Wood and others said.

Several alternative theories have been presented. Some researchers blame exposure to fine particles in air pollution, which may give the immune system more of a hair trigger, especially in genetically predisposed individuals. Others say obesity and a sedentary lifestyle may play a role. Still others wonder whether eating more processed food or foods processed in different ways, or changes in the balance of certain vitamins that can affect the immune system, such as vitamins C and E and fish oil, are a factor.

"Cleaning up the food we eat has actually changed what we're eating," said Thomas Platts-Mills of the University of Virginia.

But many researchers believe the hygiene hypothesis is the strongest, and that the reason one person develops asthma instead of hay fever or eczema or lupus or MS is because of a genetic predisposition.