Early exposure to germs in the gut has lasting benefits

To the dismay of mothers everywhere, the idea that exposure to microbes can be good for us—by tuning up our immune systems and preventing overreactions like asthma and autoimmune diseases—is catching. Now, a new study of this provocative notion, known as the hygiene hypothesis, suggests that microbes furnish some of their benefits in an unexpected way. Researchers have found that the typical intestinal bacteria in mice rein in a rare type of immune cell, curtailing asthma and colitis in the rodents.

Proposed more than 20 years ago, the hygiene hypothesis posits a downside to modern society's battle against microbes. To function properly, the hypothesis suggests, the immune system needs to tangle with microbes when we are young. Without these early interactions, our immune cells later in life become more likely to promote inflammatory and autoimmune conditions such as allergies, asthma, inflammatory bowel disease, and multiple sclerosis. The hygiene hypothesis offers an explanation for observations such as the lower frequency of asthma and allergies among children who grow up on farms: They presumably encounter more germs, or a greater diversity, than do city kids. “There is a general consensus that microbial exposure protects against the development of a variety of different allergic and autoimmune diseases,” says Anthony Horner, a pediatric immunologist at the University of California, San Diego.