“The idea is that, if you’re exposed to something over and over again, you become tolerant to it rather than have an overactive response,” Dr. Permar said. (Pediatricians used to tell parents the opposite: that they should wait until their babies are older to introduce allergenic foods. But clinical trials conducted in 2015 and 2016 showed that earlier exposure is, in fact, better, and the American Academy of Pediatrics has changed its recommendations.)
Other research suggests that more exposure to “good” bugs is better. Babies who receive antibiotics, which kill the microbes in their microbiome, are at a higher risk for developing asthma, eczema and allergies, while kids who grow up on farms and are, among other things, surrounded by animals harboring all sorts of microbes have a lower risk of developing these same conditions.
One unpublished 2019 experiment reported that babies who grow up on farms have more gut bacteria known to reduce inflammation. A theory known as the “old friends” hypothesis, developed in 2003 by British microbiologist Graham Rook as a replacement for the hygiene hypothesis, suggests that frequent exposure to harmless microbes that have co-evolved with humans help to train the developing immune system.
“You need that microbial exposure to really develop your immune system fully,” said B. Brett Finlay, Ph.D., a microbiologist at the University of British Columbia and co-author of “Let Them Eat Dirt: Saving Your Child from an Oversanitized World.”
“Our immune systems are built to be exposed to things early in life so we can then be ready for the rest of our life,” he added.
Does this mean that if your kids stay home all year, their immune systems will be doomed? No. Because when it comes to how the immune system develops, “there’s so many things to consider,” said Ruchi Singla, M.D., a pediatric allergist and immunologist at the University of Chicago Medicine. “Just as the immune system is so complex, all of the things that affect it are so complex.” Immunity is largely shaped by genetics, for instance, which means that what your children do or don’t do this year will only shape certain aspects of their immunity.
“Not a one-size-fits-all”
This is why, when researchers try to tease out how specific behaviors and choices shape immune responses, they don’t always get clear-cut answers. Dozens of studies have tried to understand the health effects of attending day care by comparing kids who go to day care with those who stay home, hoping to identify differences in rates of allergies, asthma and other immune-related conditions. Yet the studies largely conflict — if one study comes to one conclusion, another one often contradicts it. Given that immunity is shaped by so many factors, Dr. Permar said, if isolating our kids for a year or two has any effect, “it will probably be subtle.”