By Maggie Fox, Health and Science Correspondent

WASHINGTON (Reuters) - Worms may hold the secret to finding a way to prevent allergies, researchers said on Thursday.

Doctors have for years been saying a little dirt may be good for children, helping "prime" their immune systems and preventing allergies from developing.

This so-called hygiene hypothesis is based in part on the observation that people in developing countries and those who live in the countryside are much less prone to allergies than people who live in modern towns, with their sanitized floors and filtered air.

Allergies are on the rise worldwide. "Currently more than 130 million people suffer from asthma, and the numbers are increasing," Dr. Maria Yazdanbakhsh of Leiden University Medical Center in Leiden, Netherlands, and two colleagues wrote for Friday's issue of the journal Science.

Efforts to pin down a cause have failed to find an explanation, not least because, microbiologists say, all the scrubbing in the world makes barely a dent in the number of bacteria and viruses that get into the body.

But one big difference between people living in poor countries and people living in rich ones is in the number of parasites -- especially roundworms, flatworms and pinworms, known collectively as helminths -- found in their bodies.

Those parasites may be doing something to the body's immune system to help prime it, and understanding that may be the key to dealing effectively with allergies, Yazdanbakhsh said in a telephone interview.

"It's not that the hygiene hypothesis is incorrect -- it's that the immunological explanation up to now is not correct," she said. "The body needs a certain amount of challenge from pathogens. It has to reach a certain set point and if that set point is not reached, something goes wrong."

Yazdanbakhsh, who studies immunology and parasites, said helminths provoke a certain inflammatory response by the body when they infest a person or when a person is simply exposed to them without having the worms set up house.

IMPORTANT ANTIGENS

She has found some evidence the worms carry important antigens -- proteins to which the immune system responds -- that provoke a long-term response.

"They live 10 years in your body. Every day they are throwing out these antigens," she said. Perhaps some of these proteins actually slow down some components of immune system reaction, she said.

It may not be just worms, Yazdanbakhsh said. "I think viruses and bacteria, some of them can do it, too," she said.

Allergies -- and autoimmune diseases such as juvenile diabetes -- are a "friendly fire" response by the body, when it mistakenly attacks a pollen particle or a healthy cell.

Yazdanbakhsh and colleagues have some theories about what the parasites are doing that helps prevent this immune system overreaction. "We are working on some molecules," she said. "We have a class of substances that seem to be very potent in inducing anti-inflammatory responses."

Such a molecule might be developed to mimic the immune-priming effects of a parasitic infection "without paying the price of becoming infected with noxious pathogens," Yazdanbakhsh and colleagues wrote.

It might be included with childhood vaccinations, said Yazdanbakhsh, who said she treated her own 7- and 9-year old children for worms when they picked up a case.

"I shouldn't have," she joked, adding she did allow a certain amount of dirt into their lives. "My children always picked up the pacifier from the floor and put it back in their mouths," she laughed.