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 Inhaled antigens deposited in lung Stimulate antibody formation → immune complex formation (IgG) (type III hypersensitivity) Results in complement activation → inflammation (lymphocytic alveolitis) If chronic, persistent inflammation will lead to fibrosis Clinical presentation Acute Symptom onset 4–6h following high intensity exposure Examination often normal, may have crackles (no wheeze) Chronic Chronic HP results from repeated low dose antigen exposure over time Usually no history of preceding acute symptoms Insidious onset of respiratory and constitutional symptoms is typical - progressive dyspnoea, cough, and malaise On examination - may be crackles, clubbing may be present (but unusual) Investigations Acute CXR - widespread pulmonary infiltrates Chronic CXR - pulmonary fibrosis, most commonly in upper zones CT - ground glass attenuation and patchy micronodules of fibrosis in the upper lobes PFTs - restrictive (fibrosis) Bloods - serum antibodies Management Acute Chronic Oral steroids if dyspnoea or low gas transfer Antifibrotic therapy (pirfenidone, nintedanib)
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