Epidemiologic evidence from various sources suggests that exposure to microbial stimuli during early childhood can influence the induction and expression of atopic diseases, particularly in the respiratory tract. Moreover, these effects may have long-lasting consequences in relation to expression of the atopic phenotype in adulthood. This review discusses key aspects of this evidence in relation to the underlying mechanisms which regulate T-helper (Th)-cell function; in particular, the generation of Th-memory cells responsive to inhalant allergens.