The clinical and pathological findings in a fatal case of farmer's lung in a 17-year-old youth are described. His lungs were uncomplicated by chronic disease, and his first exposure to mouldy hay probably occurred only weeks before his death. As this was an acute attack, whose duration could be measured in days, the changes seen are probably the earliest to be recorded, and the probable nature of the sensitivity reaction can be deduced. The pathological process is seen to be an acute centrilobular bronchopneumonia of a special type and is associated with an obstructive bronchiolitis. There is a focal interstitial pneumonia with much proliferation of alveolar epithelium, and there is a necrotic exudate in which neutrophils and eosinophils are seen along with mononuclears, some lymphocytes, and plasma cells. The alveolar spaces are in many cases obscured, but others contain a haemorrhagic fluid. Alveolar capillaries show an acute vasculitis, with platelet and fibrin deposition, and with acute neutrophil infiltration. Serological and mycological data are included. Precipitins to mouldy hay and to Thermopolyspora polyspora were found in the serum. In dusts collected from his habitat T. polyspora was found. We think the bronchiolitis explains the severe dyspnoea, and the vasculitis accounts for the malaise, fever, and haemoptysis. The histological findings are compared with accounts of experimentally produced Arthus reactions and tuberculin reactions, and we give our reasons for believing the fundamental lesion to be an Arthus reaction.
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