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No signs of activity markers in peripheral blood despite increased bronchial reactivity after repeated low‐dose allergen exposure

The allergen inhalation test can be used as an experimental model to study pathophysiological events in allergic asthma. Repeated low‐dose inhalations of allergen induce increased bronchial hyperresponsiveness (BHR) and resemble natural allergen exposure. The objective of the present study was to investigate whether eosinophil recruitment and activation in peripheral blood, differences in expression of lymphocyte surface antigens and increased bronchial responsiveness to histamine occur during and after repeated low‐dose bronchial allergen challenge. Fourteen atopic asthmatic patients were challenged in a randomized double‐blind manner for 7 days with either allergen in very low doses or placebo. We measured the concentration of eosinophils, eosinophil cationic protein (ECP) and the expression of the EG2‐epitope on intracellular ECP in eosinophils and the expression of lymphocyte surface antigen markers in peripheral blood. The challenge period started and ended with a histamine provocation. The repeated low‐dose allergen exposure resulted in a significant increase in BHR. No changes were seen in the placebo group. Concerning the inflammatory parameters in peripheral blood, no significant changes were seen during or after the week of low‐dose allergen inhalations. Our results show that very low, repeated doses of allergen induce increased airway reactivity despite lack of evident clinical symptoms or signs of activation of inflammatory cells in peripheral blood.