Which exposures are classic causes of hypersensitivity pneumonitis?

• A. Farmers mold hay or avian proteins
• B. Cigarette smoke
• C. Ozone
• D. Silica dust

Answer: (A)

Hypersensitivity pneumonitis is driven by inhaled organic antigens that provoke immune-mediated inflammation in the lungs. Classic examples are exposures common to farming and bird handling: moldy hay contaminated with thermophilic organisms and avian proteins from feathers or droppings. These antigens trigger both immune complex–mediated and cellular immune responses in the alveoli and interstitium, leading to fever, cough, and dyspnea after exposure and potential progression with ongoing contact.

Cigarette smoke is a major risk for other lung diseases like COPD and lung cancer, not the classic trigger for hypersensitivity pneumonitis. Ozone exposure causes airway irritation rather than the immune-mediated reaction seen in HP. Silica dust leads to silicosis, a different pneumoconiosis caused by inorganic particles.