A 25-year-old farmer develops wheezing and dyspnea 4–6 hours after entering a hay barn. He has no immediate reaction on entering. Serum precipitins against thermophilic actinomycetes are positive. Lung biopsy shows non-caseating granulomas and lymphocytic alveolitis. This is BEST classified as:

• A Type I (IgE-mediated) hypersensitivity with mast cell degranulation
• B Type II (antibody-dependent cytotoxic) hypersensitivity targeting alveolar epithelium
• C Type III (immune complex) and Type IV (delayed/cell-mediated) hypersensitivity combined — hypersensitivity pneumonitis ✓
• D Type IV hypersensitivity exclusively, mediated by CD8+ cytotoxic T cells

Explanation

Hypersensitivity pneumonitis (extrinsic allergic alveolitis), exemplified here by Farmer's lung from Saccharopolyspora rectivirgula antigens, involves a combined Type III (IgG immune complex deposition in alveolar walls at 4–8 hours) and Type IV (granulomatous CD4+ Th1 response at 24–48 hours) mechanism. The delayed onset (not immediate), presence of serum precipitins (IgG), and granulomatous histology with lymphocytic alveolitis are all consistent with this dual mechanism. Type I would present within minutes with IgE and eosinophilia; Type II would show autoantibody-mediated cell destruction (e.g., Goodpasture).

Reference: Robbins & Cotran Pathologic Basis of Disease, 10th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.