Anti-GBM disease (Goodpasture syndrome) produces linear IgG deposits along the GBM. The target antigen is the NC1 domain of alpha-3(IV) collagen. Why does the disease also cause pulmonary hemorrhage in some patients but spare the lung in others despite systemic autoantibodies?

• A Pulmonary alveolar basement membrane lacks alpha-3(IV) collagen and is only involved when there is concurrent ANCA vasculitis
• B Lung involvement in anti-GBM disease depends entirely on complement C3 levels, with low C3 preventing alveolar injury
• C The alpha-3(IV) collagen NC1 epitope in pulmonary alveolar basement membrane is normally cryptic; factors such as smoking, infection, or high oxygen exposure unmask it, permitting IgG binding and complement activation ✓
• D Only anti-alpha-4(IV) collagen antibodies affect the lung; anti-alpha-3(IV) antibodies are kidney-restricted

Explanation

Both glomerular and alveolar basement membranes express alpha-3(IV) collagen, but the Goodpasture epitope within the NC1 domain is normally cryptic in the alveolar basement membrane due to collagen IV network arrangement. Environmental insults such as cigarette smoking, pulmonary infections, hyperoxia, or solvent inhalation alter the quaternary structure of the alveolar collagen IV network, exposing the epitope and allowing circulating anti-alpha-3(IV) IgG to bind and activate complement, causing diffuse alveolar hemorrhage. This explains the epidemiological observation that smokers with anti-GBM disease are far more likely to have pulmonary involvement. Complement levels are not the determining factor, and the antigen is the same (alpha-3 chain) in both organs.

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

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Written and medically reviewed by the StethoPrep medical team.