(NATURE) — Exposure to germs in childhood is thought to help strengthen the immune system and protect children from developing allergies and asthma, but the pathways by which this occurs have been unclear. Now, researchers have identified a mechanism in mice that may explain the role of exposure to microbes in the development of asthma and ulcerative colitis, a common form of inflammatory bowel disease.

In a study published online today in Science1, the researchers show that in mice, exposure to microbes in early life can reduce the body’s inventory of invariant natural killer T (iNKT) cells, which help to fight infection but can also turn on the body, causing a range of disorders such as asthma or inflammatory bowel disease.

The study supports the ‘hygiene hypothesis’, which contends that such auto-immune diseases are more common in the developed world where the prevalence of antibiotics and antibacterials reduce children’s exposure to microbes.

“We as a species are not exposed to the same germs that we were exposed to in the past,” says study co-author Dennis Kasper, a microbiologist at Harvard Medical School in Boston, Massachusetts.

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