Background: According to recent studies in children, IgE sensitization not only against perennial allergens, but also against food allergens, is related to asthma risk and increased airway inflammation. During the last decade, a new technique for IgE determination based on allergen components has become available, but its use in epidemiological studies has been limited. Aims: To investigate the relationship between the pattern of IgE sensitization to allergen components and the prevalence of asthma, airway inflammation and hyperresponsiveness in a population-based setting. To examine the relationship of IgE sensitization to allergen extract, and airway inflammation, airway hyperresponsiveness and blood eosinophilia in asthmatics. To examine the natural history of IgE sensitization to food allergens in adults. To compare extract-based and component-based IgE measurements in relation with new-onset respiratory disease and airway inflammation and hyperresponsiveness. Methods: The present thesis is based on cross-sectional and longitudinal analyses of the adult, the population-based study ECRHS (European Community Health Survey) and a cross-sectional, observational study of young subjects with asthma. IgE sensitization was examined by means of both extract-based and component-based tests. Airway inflammation was assessed by exhaled NO and airway hyperresponsiveness with methacholine test. Results: IgE sensitization to food allergens independently related to increased airway inflammation in both a population-based study and a study of asthmatics. Furthermore, a relation was found with increased blood eosinophils in asthmatics. The decrease in prevalence of IgE sensitization against food allergens during a 9-year follow-up was larger than the decrease of aeroallergens. Subjects with IgE sensitization to both cat extract and components showed more frequent airway inflammation, greater bronchial responsiveness and higher likelihood of developing asthma and rhinitis than subjects with IgE sensitization only to cat extract. Conclusions: The presence of IgE antibodies against food allergens was independently associated with airway and systemic inflammation. Both aeroallergens and food allergens should be examined in order to understand the signaling of local and systemic inflammation in asthma. Prevalence of IgE sensitization to food decreased in adults to a larger extent than IgE sensitization against aeroallergens. Measurement of IgE sensitization to cat allergen components appears to have a higher clinical value than extract-based measurement