Antiglomerular basement membrane (anti-GBM) disease with clinical and histological features that bridge the typical to atypical spectrum.

We describe antiglomerular basement membrane (anti-GBM) disease with rapidly progressive glomerulonephritis and concurrent parainfluenza pneumonia. Circulating anti-GBM antibodies were barely detectable and disappeared rapidly following corticosteroids, cyclophosphamide and plasma exchange. Kidney biopsy demonstrated strong linear GBM staining for IgG and IgG4 and unusually prominent endocapillary hypercellularity, suggesting 'atypical anti-GBM disease', although glomerular necrosis and crescents were also seen. When kidney function deteriorated further, despite persistently absent circulating anti-GBM antibodies, a repeat kidney biopsy was performed, showing crescents in 100% of glomeruli with ongoing endocapillary hypercellularity and strong IgG and IgG4 GBM staining. This case highlights complexities in the diagnosis of anti-GBM disease, with clinical and histological features bridging the atypical to typical anti-GBM disease spectrum. We hypothesise that these findings might be explained by the presence of IgG4 (rather than traditional IgG1 or IgG3) autoantibodies. To our knowledge, this is also the first report of parainfluenza associated with anti-GBM disease.