Evidence is steadily accumulating which indicates that the patterns of T cell reactivity against the environmental antigens that determine the allergen responder phenotype in adulthood are, in many cases, established during infancy. The underlying regulatory processes which determine the nature of long-term immunological memory against these antigens appear to involve a combination of classical high zone and low zone tolerance mechanisms. It appears likely that these T cell responses are initiated before birth via the transplacental transfer of low levels of allergen to which mothers are exposed during pregnancy.