Why does airway obstruction persist in asthma due to low-molecular-weight agents? A Pathologistís view.

The pathology of occupational asthma, which is similar to that of nonoccupational asthma, is characterized by airway infiltration of eosinophils, mast cells, and T-lymphocytes associated with thickening of the subepithelial reticular basement membrane. Since occupational asthma is caused by exposure to a sensitizing agent present in the working environment, it might be expected that cessation of occupational exposure leads to complete recovery from the disease. Unfortunately, this fa orable prognosis is observed in only a small percentage of patients with occupational asthma; in most of them, symptoms and bronchial hyperresponsiveness persist, although often at a decreased level. The few longitudinal studies performed on airway pathology in subjects with occupational asthma have shown that, after cessation of exposure to the sensitizing agent, some of the pathologic alterations (such as the subepithelial collagen deposition) improve, whereas others (such as the airway eosinophilia) persist. This latter finding suggests a role for eosinophils in the persistence of symptoms and bronchial hyperresponsiveness in the majority of asthmatic subjects even several months after removal from exposure.