Pediatricians used to tell parents to wait until children were toddlers before feeding them foods like peanuts and egg , common causes of food allergy. The theory was that early exposure to such allergens might be driving the development of food allergies, in which the immune system reacts to harmless food molecules as if they were dangerous. But that didn’t stop allergy rates in U.S. children from going up— by 50%, from 1997 to 2011. It turns out that giving children allergens early actually