Acute or chronic hypersensitivity type III reaction to an antigen

Aetiology

• Caused by an allergic reaction affecting the small airways and alveoli in response to an inhaled antigen or occasionally following ingestion of a causative drug
• Common antigens include thermophilic bacteria (farmer's lung), avian proteins (bird fancier's lung), and fungi (malt worker's lung)
• No cause identified in 30% of cases

Pathophysiology

1. Inhaled antigens deposited in lung
2. Stimulate antibody formation → immune complex formation (IgG) (type III hypersensitivity)
3. Results in complement activation → inflammation (lymphocytic alveolitis)
4. If chronic, persistent inflammation will lead to fibrosis

Clinical features

Acute

• Symptom onset 4–6h following high intensity exposure
• Malaise
• Dry cough
• Pyrexia
• Dyspnoea
• Examination often normal, may have crackles (no wheeze)

Chronic

• Chronic HP results from repeated low dose antigen exposure over time