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Background: Although food allergens and house dust mites are thought to contribute to the development of atopic dermatitis (AD) in infants, the influence of pollens has not yet been well defined. Objective: This study attempted to clarify the effect of Japanese cedar pollens (JCPs), which are scattered in Japan between February and April, on AD in infants. Methods: In the first study, seasonal changes in the number of patients who first visited our institute due to AD were analyzed by reviewing medical records of 184 infants with AD. In the second study, peripheral blood mononuclear cells from 81 children (50 infants <1 year and 31 older children) with AD were stimulated with a standardized JCP allergen. The production of IL-5 was measured and its correlation with the severity of AD in infants was analyzed. Results: The number of infants who first visited the institute due to AD peaked in spring. JCP-specific IL-5 production began to increase immediately after the first contact with JCPs in infants with AD. The production of IL-5 in infants with AD exposed to JCPs was significantly higher than in control subjects (p < 0.002) or in AD infants never exposed to JCPs (p < 0.02). There was a significant positive correlation between IL-5 production and the severity of AD in infants (p < 0.0001). Conclusion: These results suggest that JCPs play an important role in the aggravation of infantile AD in spring by inducing IL-5 production. Copyright © 2004 S. Karger AG, Basel [ABSTRACT FROM AUTHOR]