Renal interstitial immunoglobulin deposits Monoclonal immunoglobulin deposition is characterized by nonorganized electron-dense granular deposits predominantly localized in basement membranes of the kidney (other organs may additionally be affected). By light microscopy, tubular lesions are characterized by the deposition of a refractile, eosinophilic, periodic acid-Schiff (PAS)-positive, ribbon-like material along the tubular basement membrane. immunofluorescence examination of the kidney. Immunofluoerescence may show evidence of monotypic light or heavy chain fixation along tubular basement membranes. Electron microscopy can show finely to coarsely granular electron-dense deposits along the outer (interstitial) aspect of the tubular basement membranes. Accumulation of an immunoglobulin in the interstitial tissue of the kidney. The immunoglobulin may be a monoclonal Ig or the corresponding heavy-chain (HC) or light-chain (LC) subunit. By convention this definition excludes Ig-derived amyloidosis (amyloidosis can be distinguished by its affinity for Congo red staining). Renal interstitial immunoglobulin deposition Renal interstitial deposits