Is Henoch-Schönlein purpura the systemic form of IgA nephropathy?

Despite different clinical features, IgA nephropathy (IgAN) and Henoch-Schönlein purpura (HSP) are indistinguishable by histopathology, leading to the suggestion that HSP is a systemic form of IgAN. This review compares and contrasts the clinical, pathologic, and experimental similarities and differences of these two disorders. Many patients with HSP have minimal extrarenal disease, while up to 30% of patients with IgAN will subsequently have systemic symptoms. Although patients with HSP are usually much younger than those with IgAN, the age distributions often overlap. Both may have recurrent macroscopic hematuria associated with pharyngitis, a similar risk of developing renal insufficiency, and recurrent disease after kidney transplantation. Although the pattern of IgA subclass and complement deposition are similar, monocytic and T lymphocytic infiltrates have been observed only in HSP. Dermal blood vessels of many patients with IgAN have IgA immunofluorescence similar to that in HSP, supporting a systemic process in IgAN. Although the pathogenesis is not clearly understood for either disease, investigations of potential disease mechanisms have revealed striking similarities. These include an up-regulated in vitro IgA immune response, circulating IgA-containing immune complexes and autoantibodies, and decreased Fc receptor-mediated immune clearance. Finally, immunogenetic studies suggest that patients with both conditions inherit a predisposition for disease.