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Background: Exposure to allergens early in life may have an impact on the incidence of allergy many years later, but the kinetics of the immune responses have still not been studied prospectively. Therefore, we wanted to study the development of immune responses of the Th1 and Th2 type to birch over the first pollen seasons. Material and Methods: Blood samples were obtained from 21 prospectively followed children during the second to the seventh pollen season of life. IgG subclass antibodies to rBet v 1 were analyzed by ELISA, IgE antibodies to birch with Magic Lite™ and birch-induced mononuclear cell proliferation by 3H-thymidine incorporation. Results: Proliferative responses and IgG1 antibodies were commonly seen both in children with and without allergic symptoms and sensitization to birch. Most nonsensitized children had a transient IgG4 antibody response, which was downregulated after the third pollen season, while the titers of this Th2-associated subclass increased with age in sensitized children with clinical symptoms to birch. Conclusions: Immune responses to birch can be demonstrated in children regardless of atopic status. A transient early Th2-like response is downregulated after the third pollen season in nonatopic but not atopic children. Copyright © 2001 S. Karger AG, Basel.