If you sneeze, sniff and cough your way through one allergy season after another, you have plenty of company. As a nation, our allergies are getting worse. And researchers have developed a counterintuitive theory to explain why so many of us are miserable.

Cleaner homes and smaller families, they say, are throwing off our immune systems. In fact, researchers now say we need exposure to healthy doses of bacteria and infectious agents early in life to develop properly.

The so-called "hygiene hypothesis" holds that a developing immune system needs practice at fighting off infections. And without that practice, the immune system mistakenly targets the dust mites, pet dander, molds and pollens that cause allergies.

High rates of autoimmune disorders such as multiple sclerosis, lupus, diabetes and rheumatoid arthritis - diseases caused in part by overactive immune systems - also have been linked to the hypothesis.

"Basically, exposure to high levels of bacteria and infectious agents can rig your immune system so that you're less likely to develop allergic reactions," said Andrew Saxon, a centscm+RDaconroy:UCLA University of California, Los Angeles, researcher who studies allergies and the immune system.

A primary goal for Saxon and others investigating the hypothesis and its implications is finding a drug that will not just treat allergies, but cure them.

"We may be getting close, but we aren't there yet," he said.

Carol Conway wishes there was something she could take. For about a year, she's had itchy eyes from mold in her Phoenix home's basement, where she goes frequently to sew and do laundry.

"Right now my eyes are screaming. They itch, burn, sting and drip," said Conway, 66. "I have two red things for eyes."

Her general practitioner has told her it's allergies, but over-the-counter medications haven't helped. With her symptoms getting worse in recent weeks, she's looking for an allergist.

Doctors treat allergic patients with oral medications or periodic injections, said Dr. Sean O'Brien, a Baltimore allergist and spokesman for the Asthma and Allergy Foundation of America.

He and other allergists are busy year-round treating patients with itchy eyes, runny noses, congestion and sneezing. Right now, it's too early for ragweed pollen and too late for tree and grass pollen - but mold is right in season, O'Brien said.

O'Brien is always busy, but he said it's difficult to tell whether that's a result of the hygiene hypothesis, he said. "People are still trying to figure out why we're seeing more allergies," he said.

Surveys show that up to 50 million Americans suffer from allergies, according to the American Academy of Allergy, Asthma and Immunology. Another 20 million people have asthma. Allergens can trigger asthma attacks, and more than 70 percent of people with asthma also suffer from some type of allergy, according to the academy.

The hygiene hypothesis originated in the 1990s, when health researchers noticed a curious twist in German health statistics after the fall of the Berlin Wall. There were fewer children with asthma and allergies in the polluted areas of East Germany than in West Germany's much-cleaner communities.

But experts caution that the hypothesis is still just that.

"I think the hygiene hypothesis is one reason all forms of allergies are rising, but I'd be hesitant to say it's the primary reason," said Dr. Robert A. Wood, director of pediatric immunology at the Johns Hopkins Children's Center.

For one thing, the hypothesis fails to explain why asthma rates are higher in inner-city neighborhoods and among African-Americans. Researchers say genetics and yet-unidentified environmental factors are probably playing a part, too.

"The hygiene hypothesis makes sense and it explains some things, but it doesn't explain everything and it may be oversimplified," said Dr. Marshall Plaut, an expert at the National Institute of Allergy and Infectious Diseases.

Scientists have ample evidence that the environment plays a key role in how immune systems develop.

In June, researchers at Duke University Medical Center published findings showing that wild rats captured from farms and fields in North Carolina had much more robust immune systems than the same species of rat raised in the lab, with higher levels of a type of antibody that rodents - and humans - use to respond to allergens.

"The differences were so profound, it was almost like they were an entirely different species," said William Parker, an assistant professor of experimental surgery and a senior member of the research group.

Parker is seeking a grant from the National Institutes of Health to create a controlled environment - a kind of barnyard for rats - to study the development of the rat's immune system.

"We want to be able to expose rats to everything they'd normally be exposed to in the wild, but still be able to observe them," Parker said.

Research also has shown that children exposed to germs early in life - by having pets in the home, being raised on a farm or being exposed to other children in homes or day care settings - have a better chance of avoiding allergies.

Scientists followed 474 youngsters in Detroit during the first six years of their lives and found those raised with two or more dogs or cats in the house were 70 percent less likely to become allergic to pets than children in homes without those pets. In fact, children in homes with pets had fewer allergies of all kinds.

"We found lower allergy rates for every allergen we tested them for," said Dr. Dennis Ownby, chief of the Medical College of Georgia's allergy and immunology section. He published the research in the Journal of the American Medical Association in 2002, when he was a staff physician at the Henry Ford Hospital in Detroit

The pets carry bacteria that release molecules called endotoxins, which likely have a positive effect on the immune system, Ownby said. The researcher continues to follow the children to see how pet exposure affects allergy rates as the youngsters grow up.

Researchers have also found that infants in day care centers were less likely to develop allergies later in life than those who weren't. The findings were published several years ago in the New England Journal of Medicine.

"Probably the crucial period is the first six months of life," said Dr. Fernando Martinez, director of the Arizona Respiratory Center at the University of Arizona's College of Medicine.

He's working with European investigators to explore the genetic traits that make some farm children in Germany, Switzerland and Austria more likely than others to avoid allergies.

The immune system uses a type of receptor, known as a toll receptor, to recognize harmful bacteria. In work so far, Martinez has found that a genetic variant can affect the number of toll receptors a person has and whether they are likely to develop asthma or allergies.

For some children, it's as if they were never raised on a farm," he said.


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