Researchers have long known that growing up on a farm seems to protect children against allergies and now they know the secret lies in the dust, Belgian experts said Thursday.

The findings published in the US journal Science could help lead to a vaccine against asthma one day.

"At this point, we have revealed an actual link between farm dust and protection against asthma and allergies," said Bart Lambrecht, a professor of pulmonary medicine at Ghent University.

"We did this by exposing mice to farm dust extract from Germany and Switzerland. These tests revealed that the mice were fully protected against house dust mite allergy, the most common cause for allergies in humans."

Scientists also discovered that farm dust "makes the mucous membrane inside the respiratory tracts react less severely to allergens such as house dust mite" due to a protein called A20, the study said.

The body produces the A20 protein when a person comes in contact with farm dust.

Professor Hamida Hammad at Ghent University said the protective effect went away when the A20 protein was inactivated in mice, leaving the mucous membrane of the lungs "unable to reduce an allergic or asthmatic reaction."

When researchers examined a group of 2,000 people who grew up on farms, they found most did not suffer from allergies or asthma.

Those who were still prone to allergies and asthma were found to be deficient in the protective protein.

"Those who are not protected and still develop allergies have a genetic variant of the A20 gene which causes the A20 protein to malfunction," said Lambrecht.

Next, researchers will be hunting for the active substance in farm dust that is responsible for providing protection, so that they can use it to develop a preventive medicine against asthma.

"Discovering how farm dust provides this type of protection has certainly put us on the right track towards developing an asthma vaccine and new allergy therapies," said Hammad.

"However, several years of research are required still before they will be available to patients."