Kids who grow up on farms are far less likely to develop asthma or allergies compared with the average child — and now scientists think they know why.

A study published in Science today shows that bacteria found in farm dust trigger an immune system response in the lungs of mice. That response seems to be why those mice are protected from allergies and asthma in following weeks. The study also suggested there was a genetic component to the protective benefit.

17 percent of kids have respiratory allergies

Respiratory allergies are the most common form of allergy in American children; about 17 percent of kids in the US suffer from these types of allergies. And about 6.8 million American children currently have asthma. But studies show that these conditions aren't distributed uniformly among all children. Children raised on farms — most notably dairy farms — have lower rates of allergies and asthma. That's why researchers are looking into the farm environment itself; if they can decipher the mechanism that's protecting kids who grew up in rural areas, they might be able to engineer drugs that can help those that live in more urban environments, too.

In the study, researchers exposed mice to endotoxins — fragments of dying bacteria found in farm dust and cow manure — for about two weeks. Then, the scientists tried to trigger a dust mite allergy in the rodents. They found that mice that had been exposed to the endotoxins were protected from the induced allergy — those who hadn’t been exposed developed asthma in response to dust mites. Upon closer examination, the researchers found that a molecule in the lung tissue, called A20, seemed to play a role in regulating the immune responses in the cells of the mice who’d been exposed to farm dust.

To confirm A20 was involved, researchers created mice that didn’t have the molecule in their lungs. When these mice were exposed to farm dust or to pure endotoxins, they still developed asthma later. So the protective response from farm dust can’t be triggered without high enough levels of A20.

Farm kids with allergies have a genetic variant that causes A20 to malfunction

Then the researchers turned to humans, exposing human lung cells to endotoxins. They found that the exposure lowered the immune response in culture samples drawn from healthy people. But the same exposure didn’t lower the response as much in cell cultures drawn from people with asthma. In addition, cells from people with asthma didn’t produce as much of the A20 molecule.

Finally, the researchers used data from 1,700 European farm children, ages 6 to 12, collected in another study. They found that farm kids who suffered from respiratory allergies had a genetic variant that causes the A20 molecule to malfunction.

"We would've predicted that farm dust would change the immune system, but in fact, it's not working directly on the immune system, it's working on the structural cells of the lungs," says Bart Lambrecht, a pulmonologist at Ghent Univerisity in Belgium and a co-author of the study. When these cells are exposed to farm dust, they get "numbed or cooled off," which causes them to stop recognizing common allergens later in life. "In this way, you don't develop allergies anymore," Lambrecht says. But that only seems to be true if a person has high enough levels of A20 activity in their lungs. "It's an entirely new mechanism by which we can explain how farm dust protects from allergies," Lambrecht says.

"It's working on the structural cells of the lungs."

"This gives us a tantalizing molecular mechanism for understanding the epidemiological evidence," Stuart Turvey, a pediatric immunologist of the University of British Columbia who didn't work on this study, told Science News. But not everyone is as enthusiastic; William Cookson, asthma geneticists at Imperial College London, told Science News that endotoxin levels aren't that much higher in farm environments compared with cities. That means that microbial changes in the gut and in the lungs likely also play an important role. The study's conclusions are "too simple of an answer," he says.

A drug that could "re-educate" cells in children's lungs

The researchers would now like to identify the various elements in farm dust — elements besides endotoxins — that allow this response to take place. If they can do that, they might be able to create a drug that parents could give to young children to "re-educate the cells in their lungs" to stop them from reacting to allergens, Lambrecht says.

A drug like that probably wouldn't work in adults or children older than two or three, however. Studies have shown that people who already have asthma don't benefit from exposure to endotoxins — they actually tend to get worse, Lambrecht says. "The higher the level of endotoxin you're exposed to, the more unstable your asthma becomes."

A vaporizer that stops city toddlers from developing asthma or allergies is still a very long way off. In the meantime, the study shows that exposing kids to farm environments early in life is probably a "good idea," Lambrecht says. "In Holland and in Germany, there's a lot of farms who are now organizing daycare centers," he says. "It's a crazy idea, but it's happening as we speak in Western Europe; this study shows that we shouldn't be afraid to do that — to send our kids to farms for daycare."