The incidence of food allergy is rapidly increasing, creating a growing public health concern. The Centers for Disease Control documented an 18% increase in only 10 years (Branum and Lukacs, 2008). This rapid change in such a short time suggests that gene-by-environment interactions are involved. All barrier surfaces of the human body are colonized with symbiotic microbial communities that interface with the external environment. These microbes aid the host by providing producing nutrients and stimulating the education and maturation of the immune system. The composition of this colonizing microbiota can be strongly influenced by environmental and lifestyle factors including diet, use of antibiotics, and mode of birth (Feehley et al., 2012). We hypothesized that changes to the microbiota may alter susceptibility to sensitization with food allergens, leading to the increased incidence of allergy. We found that treatment with broad-spectrum antibiotics (Abx) dramatically altered the composition and number of bacteria in the intestines of wild type (WT) specific-pathogen free (SPF) mice and lead to increased peanut (PN)-specific IgE and IgG1 responses after sensitization. This increased PN-specific response correlated with a decreased proportion of Foxp3+ regulatory T cells (Tregs) in the colonic lamina propria (LP) and IgA in the feces. We also examined the response to sensitization in mice deficient in Toll-like receptor (TLR) 4 or TLR2 to determine if an inability to sense certain members of the microbiota could similarly increase PN-specific responses. In agreement with previously published work (Bashir et al., 2004), Tlr4-/- mice had increased levels of PN-specific IgE and IgG1 compared to Tlr4+/- littermates. Tlr2-/- mice, however, had minimal responses to sensitization. We next used a germ free (GF) and gnotobiotic (“known biota”) model to identify members of the microbiota that were protective against sensitization. Much like Abx-treated mice, GF mice, which are devoid of any colonizing bacteria, had strong responses to sensitization as well as impaired colonic Treg differentiation and IgA production compared to SPF controls. Colonization of GF WT mice with a single species, Bacteroides uniformis, did not alter PN-specific responses, but colonization with a consortium of spore-forming Firmicutes from the Clostridia class was able to block the generation of a PN-specific IgE response as well as restore the Foxp3+ Treg compartment of the colon and IgA production to the levels seen in SPF mice. Abx-treated mice colonized with a Clostridia-containing microbiota were also protected from sensitization to PN, but transfer of Foxp3+ Tregs into Abx-treated mice was not sufficient to recapitulate this protection. Clostridia are unique because they reside close to the host intestinal epithelium, contributing to their protective effect. A microarray on intestinal epithelial cells (IECs) from GF, B. uniformis-, or Clostridia-colonized mice revealed that Clostridia stimulated expression of IL-22 regulated genes. IL-22, a known barrier-protective cytokine, was both necessary and sufficient to reduce intestinal permeability to food antigens, as measured by the amount of PN protein detected in the serum after oral administration. Neutralizing IL-22 ablated the protective effect of colonization and increased PN-specific antibody responses. Experiments to determine how Clostridia signal to the host have implicated the production of the short chain fatty acid (SCFA) butyrate in mediating protection. Treatment of GF mice with butyrate increased expression of Il22 and ex vivo culture experiments have confirmed this finding. Preliminary data suggests that IL-22 production occurs through G-protein coupled receptor (GPCR) signaling after butyrate stimulation. Experiments to directly link butyrate, GPCRs, and responses to sensitization are currently ongoing. Taken together, the results presented in this thesis demonstrate that the microbiota can profoundly alter responses to food antigens. By identifying specific protective populations and their mechanisms of action, we have provided novel targets for therapeutics that we hope will promote and maintain tolerance in patients suffering from this disease.