The early postnatal period has been identified as a time of increased risk lor primary sensitisation to aeroallergens. The expression of the allergic phenotype is predominantly genetically determined but is influenced by a myriad of environmental factors. The underlying mechanisms for allergic sensitisation to inhalant allergens have been investigated in both humans and experimental animal models. Data from the literature in both these areas are in agreement that the nature of the initial response of the T-cell arm of the immune system to first encounters with an aero-allergen can potentially determine whether allergic sensitisation will occur and be manifest in later life as allergic respiratory disease. The combination of exposure to environmental “risk factors” along with the immaturity of the mucosal component of the infant's immune system may provide a basis for the increased risk of allergic sensitisation in early childhood.