The microbiome plays a critical role in the development of the immune system and alterations in the microbiome diversity almost certainly play an important role in the surge of allergic and autoimmune diseases in 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 been associated with a major reduction in the risk of atopic diseases. Understanding the development of the immune system 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 for designing 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 lifestyle associated 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 a very 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 models indicate the importance of microbial diversity in the induction of this secretory IgA. Early studies suggest that the predisposition to the development of IgE antibodies (“atopy”) is associated with a slow development of IgA responses, 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 in established food allergy. Because there is an intimate reciprocal development of gut microbial communities with the IgA repertoire, we hypothesize that mucosal exposure to a diverse and immunogenic microbiome accelerates the development of IgA secretory immunity, counteracts development of IgE responses, and protects from allergic diseases. We will assess how B cell subsets, immunoglobulin repertoire and somatic mutation rates (Aim 1), and gut microbiome 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 other allergic diseases and humoral responses to allergens (Aim 3). In summary, this proposal will determine whether 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 responses with clinical tolerance, i.e. protection from early-onset food allergy and eczema.