IgA-containing immune complexes may play a major role in IgA nephropathy, and the author emphasizes that impairment of their clearance and of hepatobiliary transport has etiological significance. In addition, an abnormality of the phagocytic system, such as mesangial cells, may lead to mesangial deposition of preformed circulating IgA-containing immune complexes. The IgA bound to the glomerular mesangium is polymeric IgA to microbial or dietary antigens in the mucosa. Mesangial cells and endothelial cells in the glomeruli may play an important immunoregulatory role in IgA nephropathy.