The microbiome plays a critical role in the development of the immune system and alterations in themicrobiome diversity almost certainly play an important role in the surge of allergic and autoimmune diseasesin developed nations that began in the 1950's and continues today. Living on farms, avoiding antibiotics,vaginal delivery, and other environmental factors leading to greater diversity in the microbiome have beenassociated with a major reduction in the risk of atopic diseases. Understanding the development of the immunesystem in populations at low risk for allergy compared to high-risk populations, developing biomarkers for?protective? immune development, and assessing immune responses to the microbiome are fundamental fordesigning and assessing future interventions. In this proposal, we will compare the Old Order Mennonites(OOM) with a very low risk for food allergies (<1%), other allergic diseases and asthma and a lifestyleassociated with a diverse microbiome (e.g. growing up on a farm, consumption of raw milk, large families,home deliveries, low rate of antibiotic use), and neonates born to families with food-allergic children with avery high risk for developing atopic diseases, e.g. a 15-20% risk of food allergy in the first year of life.We hypothesize that accelerated development of IgA mucosal immunity will be a biomarker for ?protective?immune development. Abundant IgA-coated fecal bacteria are seen in early life and data from animal modelsindicate the importance of microbial diversity in the induction of this secretory IgA. Early studies suggest thatthe predisposition to the development of IgE antibodies (?atopy?) is associated with a slow development of IgAresponses, e.g. our own data shows a delay in development of specific IgA in cow's milk-allergic infants.Exciting new discoveries further support the role for specific IgA come from trials of orally induced tolerance inestablished food allergy. Because there is an intimate reciprocal development of gut microbial communitieswith the IgA repertoire, we hypothesize that mucosal exposure to a diverse and immunogenic microbiomeaccelerates the development of IgA secretory immunity, counteracts development of IgE responses, andprotects from allergic diseases. We will assess how B cell subsets, immunoglobulin repertoire and somatic mutation rates (Aim 1), and gutmicrobiome and IgA antibody responses to microbiome develop in cohorts of high- and low-risk for allergy(Aim 2), and how these B cell biomarkers and gut microbiome relate to the development of food and otherallergic diseases and humoral responses to allergens (Aim 3). In summary, this proposal will determinewhether accelerated development of IgA responses is a biomarker for ?protective? immune development,identify fecal bacteria inducing IgA responses, and assess the association of specific IgA immune responseswith clinical tolerance, i.e. protection from early-onset food allergy and eczema.