Environmentally induced changes in the composition of the intestinal microbiota have been implicated in allergic sensitization. In this report, Nagler and colleagues show that mice that have been treated with antibiotics have an altered gut microbial diversity and increased sensitization to food allergens. The selective colonization of mice devoid of commensal microorganisms with Clostridia spp. provided protection against dietary allergens. Moreover, the authors show that the induction of IL-22 by Clostridia spp. in the intestine decreased the access of a peanut allergen to the bloodstream and led to reduced serum allergen concentrations. Thus, Clostridia spp. could potentially have a beneficial effect on the treatment of food allergies.