Doctors have noticed that as people become more urbanized, they also become more likely to suffer from allergies. To explain this, researchers have proposed what they call the "hygiene hypothesis" ? the idea that being exposed to dirt and bacteria when young actually protects people from developing allergies. As Rose Hoban reports, a new study seems to support that idea.

Bianca Schaub is a pediatrician at the University Children's Hospital in Munich, Germany. Schaub says she knew that children born into farm families have lower rates of asthma and allergies, a phenomenon that's consistent with the hygiene hypothesis. So she recruited two groups of children to test it out, but with a twist. She actually started studying the children before they were born, when their mothers were pregnant.

"One group was where the mothers were exposed to the farms during the whole pregnancy," Schaub says. "The other group was where the mothers were not exposed to farms. And we compared the immune regulation of these two groups of children."

When the children were born, Schaub took samples of blood from the umbilical cord. She found differences in their blood right away. "These children had a specific type of immune response in the sense that they had higher regulatory T cells." Those cells keep the immune system healthy, she explains, "so that there is no modification, no disease happening. So it seems like this exposure contributes to a better regulatory T cell function in early life."

Schaub says as these children grew up, they had fewer allergies and were less likely to get asthma. The effect was greatest for children of women who worked closely with animals.

"We know when they are, for example, exposed to different species of animals, that this is protective," Schaub says. "So it seems like the different species represent a variety of stimuli for the immune system, which seem to be good in terms of preventing these allergic immune responses."

Schaub says early exposure to allergens is a little like getting a vaccination when you're young. She says in the future, information from her research might be used to create a vaccine against allergies for children who don't grow up on farms, but she says that's still far off.

Schaub presented her research at the American Thoracic Society's annual meeting in San Diego, California.