For the roughly one in ten American adults navigating the quiet vigilance of food allergy, a small but carefully observed trial at Boston Children's Hospital has opened a door that was not previously known to exist. Researchers found that transplanting gut bacteria from healthy donors into severely allergic adults shifted the immune system toward tolerance in 40% of participants — not through exposure and desensitization, but through the body's own microbial architecture. The mechanism, traced to a bacterial genus called Bacteroides and the bile acid metabolites it produces, suggests that what