The pathogenesis of mesangial immunoglobulin A nephropathies was elucidated through the analysis of their clinical features and investigation of their biological characteristics. An alteration of the mucosal immune system is currently considered chiefly responsible for these diseases. A dysregulation of other compartments of the immune system is likely to enhance this abnormality and/or contribute to the persistence of immune complexes. Little is known, however, about the etiology of these renal diseases, and their treatment remains palliative or preventive.