Health experts have known for years that kids who grow up on a farm have fewer incidences of allergies and asthma than city kids. Now they might finally know why. And that bit of information might be the clue they need to develop a vaccine for asthma and better treatment strategies for allergies.

A new study published in the New England Journal of Medicine compared 30 Amish children and 30 Hutterite children from two farming groups in North Dakota. Researchers chose these children because asthma is rare among the Amish but common among the Hutterites, even though the groups have similar genetic backgrounds and diets and very little exposure to tobacco smoke, polluted air and indoor pets. The one key difference lies in their farming methods: The Amish shun electricity and industrialization, while the Hutterites embrace it. And because of this, the children are exposed to different microbes.

“We never thought we would see a difference,” Carole Ober, an author of the study and the chairwoman of the department of human genetics at the University of Chicago, told the New York Times. But to their great surprise, “we saw whopping differences with very, very different cell types and cell numbers.”

The Amish children all had a large proportion of neutrophils — white blood cells that are part of the so-called innate immune system. The Amish kids' neutrophils "were newly emerged from their bone marrow, evidence of a continual low-grade reaction to microbial invaders," the New York Times reports. In contrast, the Hutterite kids had "old" neutrophils, and researchers found their blood was full of another type of immune cell, eosinophils, which provoke allergic reactions.

"I keep saying if everyone would just put a cow in their house, no kid would have asthma, but that's not very practical," Ober told Live Science. Instead, Ober envisions an air mister parents could use to spray the beneficial microbes into the air.

On farms and allergies

In 2015, researchers at two Belgian universities collaborated on a study that looked at the relationship between farm living and asthma and allergies. They found that a bacteria detected in farm dust might initiate an immune system response that reduces the body's reaction to allergens and asthmatic triggers.

According to the Centers for Disease Control and Prevention, 6.8 million children in the U.S. have been diagnosed with asthma and about 17 percent of kids suffer from respiratory allergies to things like pollen and mold. But over the last few decades, health care providers have observed that kids who grow up on farms are much less likely to be affected by these conditions than their urban-dwelling peers.

So researchers have been looking at farm dust to better understand what it is about it that protects kids from respiratory problems. For the 2015 study, researchers exposed mice to bacteria found in farm dust. After two weeks, they exposed those mice to dust mites. They found that the mice who had been exposed to the bacteria were less likely to have an asthmatic or allergic reaction to the dust mites than the mice who were not exposed.

What's more, researchers found that when the mice were exposed to farm dust, they produced a protein in their lungs called A20. When they deactivated this protein, these mice had a normal allergic response to the dust mites.

Researchers confirmed these findings with human surveys where they found that people who suffer from allergies and asthma have a deficiency in their A20 protein. Finally, they took a look at the kids growing up on farms who develop asthma and allergies. You guessed it — those kids have a mutation in their A20 gene that impedes the production of the A20 protein.

By narrowing down the mechanisms behind certain asthmatic and allergic reactions, these researchers may have opened the door for the development of vaccines and treatment options for people who suffer from these conditions.

Editor's note: This story has been updated since it was originally published September 2015.