Increases in airway eosinophils occur during the late asthmatic response, 7 hours after allergen inhalation, and these can persist for 3 days. Also, increases in airway metachromatic cells occur which are most marked after 7 hours. These increases in airway cells are associated with increases in bone marrow progenitors, which are caused by an increased responsiveness of the bone marrow to IL-5 after allergen because of an increased expression of the IL-5 receptor on the progenitors. These studies suggest that after allergen inhalation, signals are sent from the airways to the bone marrow, which increase production of progenitors and make more cells available to be recruited into the airways.