Hypersensitivity pneumonitis is an immune-mediated lung disease in which the repeated inhalation of certain antigens provokes a hypersensitivity response, with granulomatous inflammation in the distal bronchioles and alveoli of susceptible people. A diverse range of antigens including bacteria (Thermophilic actinomycetes), fungi (Trichosporon cutaneum), animal proteins (bird antigens), mycobacteria, and chemicals may cause the disease. The commonest forms are bird fancier’s lung, farmer’s lung, humidifier lung, and metal-working fluid pneumonitis. In some cases no antigen is identified. Acute disease is characterized by recurrent episodes of breathlessness, cough, fevers, malaise, and flu-like symptoms occurring 4–8 hours after antigen exposure. Fever and basal crackles are the main physical signs.
Chronic disease is characterized by the insidious development of dyspnoea and persistent pneumonitis, sometimes progressing to lung fibrosis. Clinical features are similar to those of other varieties of pulmonary fibrosis, but clubbing is uncommon.
Features of humidifier lung and comparison with summer‐type hypersensitivity pneumonitis