What is the classic immunofluorescence pattern seen in anti-GBM disease (Goodpasture syndrome) involving the glomerular basement membrane?

• A. Granular IgG and C3 deposition along the GBM.
• B. Linear staining of the glomerular basement membrane.
• C. Mesangial IgA deposition.
• D. Subepithelial immune complex humps on EM.

Answer: (B)

The key feature is a linear pattern of IgG (often with C3) along the glomerular basement membrane on immunofluorescence. This occurs because the autoantibodies in anti-GBM disease bind uniformly to antigens present across the entire basement membrane, producing a smooth, continuous line rather than discrete deposits. This linear staining is distinctive and helps separate anti-GBM disease from immune complex–mediated conditions, which show granular deposits. The other patterns align with different diseases: granular IgG/C3 along the GBM corresponds to immune complex GN, mesangial IgA deposition points to IgA nephropathy, and subepithelial humps on EM are typical of post-streptococcal (post-infectious) GN.