Background-Airway dendritic cells (DQ play an important role in chronic allergic airway inflammation in experimental animals, but a similar role for DC in human allergic asthma has been difficult to define. This pilot study was undertaken to elucidate the role of DC in allergic asthma by examining their potential to migrate to the lower airways in response to bronchial challenge with specific allergen.Methods-Bronchial biopsy specimens were obtained from seven patients with allergic asthma before and 4-5 hours after allergen challenge. Multicolour immunofluorescence staining was performed on mucosal cryosections to identify changes in the number and phenotypes of DC.Results-A dramatic increase in the number of CD1c+HLA-DR+ DC were observed in the lamina propria after challenge compared with baseline (22.4 v 7.8 cells/mm(2)). The rapid accumulation (within 4-5 hours) of these cells strongly suggests that they were directly recruited from peripheral blood.Conclusion-We have shown for the first time that a specific DC subset rapidly emigrates into the human bronchial mucosa during allergic inflammation. While this study is based on relatively few patients, the consistency of the overall results strongly suggests that the rapid population dynamics of human airway DC closely parallel those in animal models of acute inflammation. These findings support suggestions that DC have an important role in human airway allergy.