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Researchers in Europe think they have figured out how dirt might help protect kids from developing allergies and asthma.

They found that dust from dairy farms stimulates an immune response in the lungs of mice that appears to protect against asthma later in life. And they identified a compound made by cells that seems to control this process.

Kids who grow up on farms and develop asthma anyway have a genetic mutation that affects production of the protective compound, they report in their study, published in the journal Science.

“Growing up on a dairy farm protects children from allergy, hay fever, and asthma."

It may not be the last word on allergies but it gives scientists a big clue, and might help explain the so-called hygiene hypothesis. The researchers think it might lead to a way to vaccinate kids so they’re less likely to develop allergies and asthma.

Researchers have known for years that kids can be too clean. Children who live in rural areas are less likely to develop allergies and asthma, for instance.

“Growing up on a dairy farm protects children from allergy, hay fever, and asthma,” Bart Lambrecht of Ghent University Hospital in Belgium and colleagues write in their report.

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So they tested dust from dairy farms in Europe. It’s full of all sorts of bacteria and fungi.

Researchers have never been able to find one particular germ, fungus or parasite that explains the hygiene hypothesis, although they’ve looked hard. The European team couldn’t find anything specific, either.

They injected small pieces of farm dust bacteria called endotoxins into the noses of mice and found this set off an immune reaction, in the form of inflammation. Mice that reacted this way did not develop asthma later, even when exposed to house dust mites — a notorious trigger for asthma.

“We have revealed an actual link between farm dust and protection against asthma and allergies,” Lambrecht said in a statement.

“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."

The cells on the surface of the lungs produced an enzyme called A20 that was important to the response. The researchers found that mice with a mutated version of A20 were not protected from asthma.

Then, they tested 2,000 children, including some who grew up on farms but who developed asthma, anyway. Many of the asthma patients had a version of A20 that was less effective than the kids who didn’t get asthma.

Lab tests showed that A20 in human lung tissue was involved in the immune system over-response to house dust mites, they wrote.

“We have revealed an actual link between farm dust and protection against asthma and allergies."

“The hygiene hypothesis states that the rise in allergy and asthma that has been observed in affluent countries since the Second World War is caused by reduced ‘infectious pressure’ from the Western lifestyle environment,” the researchers wrote.

“The mechanism behind this association has been linked to an imbalance in the immune system,” they added.

They think they’ve found at least one component. While they tested A20 in the lungs, they said cells elsewhere in the body, in the gut for instance, also produce A20. Now tests are needed to see if it’s involved in food allergies.

They also want to find the precise components of farm dust that are triggering the reactions.