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Let Them Eat Dirt

Early exposure to microbes shapes the mammalian immune system by subduing inflammatory T cells.

Maybe it’s okay to let your toddler lick the swing set and kiss the dog. A new mouse study suggests early exposure to microbes is essential for normal immune development, supporting the so-called “hygiene hypothesis” which states that lack of such exposure leads to an increased risk of autoimmune diseases. Specifically, the study found that early-life microbe exposure decreases the number of inflammatory immune cells in the lungs and colon, lowering susceptibility to asthma and inflammatory bowel diseases later in life.

The finding, published today (March 21) in Science, may help explain why there has been a rise in autoimmune diseases in sterile, antibiotic-saturated developed countries.

“There have been many clues that environmental factors, particularly microbiota, play a role in disease risk, but there’s very little information about when it’s critical for that exposure to take place,” said Jonathan Braun, chair of pathology and laboratory medicine at the David Geffen School of Medicine at the University of California, Los Angeles, who was not involved in the research. “This is one of the most compelling observations to pin down that time frame.”

The mammalian immune system is dramatically influenced and shaped by exposure to microbes throughout life. Epidemiological evidence suggests that early-life exposure to bacteria may be key in preventing two immune diseases: asthma and ulcerative colitis, a type of inflammatory bowel disease.

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