IgA nephropathy (IgAN) is initiated by glomerular deposition of polymeric IgA1(pIgA1). In IgAN pIgA production is reduced in the mucosal immune system, perhaps mediated by a mucosal gamma delta T cell defect, and mucosal IgA responses to immunisation are impaired. But pIgA1 production by the marrow is increased. Human pIgA1 has an O-glycosylated hinge region unique to circulating immunoglobulins and there is reduction of galactosyl residues in the hinge of serum IgA1 in IgAN and Henoch-Schönlein nephritis. This reduced galactosylation may be due to a functional defect in plasma cell beta 1,3-galactosyltransferase. Altered hinge region glycosylation may alter IgA1 structure, modifying interactions with matrix proteins, IgA receptors and complement, and therefore influence mesangial deposition and subsequent injury through mechanisms other than classical antigen-antibody reactions. IgA clearance through the hepatic asialoglycoprotein receptor or Fc alpha receptors on circulating white cells may also be impaired. The unique features of human IgA1 have prevented development of satisfactory animal models for the early stages of IgAN. It is likely that events after pIgA1 deposition which result in glomerular inflammation and scarring are not specific to IgAN but generic to many forms of glomerulonephritis.