Ref: Robbins Pathologic Basis of Disease, 8th editionExplanation:RAPIDLY PROGRESSIVE (CRESCENTIC. GLOMERULONEPHRITIS (RPGN)RPGN is form of severe glomerular injury which does not denote to a specific etiologyit is characterized clinically bv rapid and progressive loss of renal function associated with severe oliguria and signs of nephritic syndromeIf untreated, death from renal failure occurs within weeks to months.The most common histologic picture is the presence of crescents in most of the glomeruli (crescentic glomerulonephritisThese crescents are produced by the proliferation of the parietal epithelial cells lining Bowman capsule and by the infiltration of monocytes and macrophages.Classification of RPGNTYPE I (ANTI-GBM ANTIBODY)Renal limitedGoodpasture syndromeTYPE II (IMMUNE COMPLEX)IdiopathicPost-infectious glomerulonephritisLupus nephritisHenoch-Schonlein purpuraIgA nephropathyTYPE III (PAUCI -IMMUNE)ANCA-associatedIdiopathicWegener granulomatosis Microscopic polvangiitisFirst type of RPGN is anti-GBM antibody- induced disease.It characterized by linear deposits of IgG and. in many cases. C3 in the GBM that are visualized by immunofluorescence.In some of these patients, the anti-GBM antibodies cross-react w ith pulmonary alveolar basement membranes to produce the clinical picture of pulmonary hemorrhage associated with renal failure (Goodpasture syndrome).Plasmapheresis to remove the pathogenic circulating antibodies is usually part of the treatment, which also includes therapy to suppress the underlying immune response.Second type of RPGN is due to immune complex deposition.It can be a complication of any of the immune complex nephritides, including postinfectious glomerulonephritis, lupus nephritis, IgA nephropathy, and HenochSchonlein purpura.In all these cases, immunofluorescence studies reveal the granular pattern of staining characteristic of immune complex deposition.This type of RPGN frequently demonstrates cellular proliferation within the glomerular tuft, in addition to crescent formation.The third type of RPGN, is defined by the lack of anti-GBM antibodies or immune complexes by immunofluorescence and electron microscopy.Most patients with this type of RPGN have circulating anti neutrophil cytoplasmic antibodies (ANCAs) that produce cytoplasmic (c) or perinuclear (p) staining patternsMorphologyThe kidneys are enlarged and pale, often with petechial hemorrhages on the cortical surfaces.Depending on the cause, the glomeruli may show focal necrosis, diffuse or focal endothelial proliferation, and mesangial proliferation.The histologic picture is characteristics by the presence of distinctive crescents.Crescents are formed by proliferation of parietal cells and by migration of monocytes and macrophages inti} the urinary space.The crescents eventually obliterate Bowman space and compress the glomerular tuft.Fibrin strands are frequently prominent between the eel hilar layers in the crescents.The escape of fibrinogen into Bowman space and its conversion to fibrin are an important contributor to crescent formation.