Allergic diseases are among the fastest growing chronic conditions in Australia, where allergy rates are among the highest in the world with 20% of the population affected. The first allergen encounter occurs early in life. Infancy is a critical period in which imbalance in the microbiome affects the establishment of allergen tolerance, predisposing to sensitisation and the development of allergy. This is defined as the “window of opportunity”. The high prevalence of these diseases in Western counties may be partially attributed to the consumption of a “Western-type diet”, which is characterized by low fibre, high saturated fat and refined carbohydrates. This diet is associated with a less diverse gut microbiome, impaired gut barrier integrity and a systemic pro-inflammatory immune phenotype likely affecting the effector/regulatory T cells equilibrium facilitating allergen sensitisation. To better understand the impact of diet on the developing immune system, we exposed male and female mice to a Mediterranean (high in fibres) or a Western diet prior to time-mating through to the weaning of the pups. The phenotypic characterisation of dendritic cells and regulatory T cells was performed in the lung, small intestine lamina propria and mesenteric lymph nodes cells of neonates on days 7, 14, and 21 post-partum. We have found that diet induced subtle homeostatic alterations in the mucosal tissues, however a striking difference involving the retinoic acid metabolism was found in the mesenteric lymph nodes. Our preliminary results indicate that maternal diet may contribute to the loss of tolerance in neonates and the development of allergy during the window of opportunity.