“All 30 of the Amish kids had this,” said Anne I. Sperling, another author of the study and an associate professor of immunology and medicine at the University of Chicago.

By contrast, six of the 30 Hutterite children had asthma, and all of them had far fewer neutrophils in their blood. The neutrophils that they did have were older ones, not cells that had just emerged. Instead, their blood was swarming with another type of immune cell, eosinophils, which provoke allergic reactions. It was as if they were primed for an asthma attack as soon as they breathed something to set it off.

With the Amish children, Dr. Sperling said, it would clearly take a lot more provocation to set off an allergic response.

The researchers decided that the differences between the Amish and the Hutterite children were so great that they should forge ahead with additional research to try to figure out what was stimulating the Amish innate immune system.

They analyzed dust from the Amish and the Hutterite homes. The Amish dust was loaded with debris from bacteria; the Hutterite dust was not. The researchers sent the dust to Dr. Donata Vercelli, an associate director of the asthma and airway research center at the University of Arizona, who would test the dust in mice.

She put dust — Amish or Hutterite — into the airways of mice 14 times over a month and then exposed the animals to allergens. She measured how the airways responded: Did they constrict and twitch? Were they inflamed?

“We found exactly what we found in the children,” Dr. Vercelli said. “If we give the Amish dust, we protect the mice. If we give the Hutterite dust, we do not protect them.”