To investigate the role of dietary components in immunoglobulin A mesangial nephropathy (IgAGN), this study focused on gliadin, based on the reported association between coeliac disease and IgAGN as well as the pilot observation that a gluten-free diet was able to reduce the levels of circulating IgA immune complexes (IgAIC). IgA mesangial deposits in mice were induced by oral immunization with gliadin and in rats by inducing alcoholic liver cirrhosis, which increased the levels of IgA against dietary antigens (Ag). Gliadin was able to bind to cultured mesangial cells by a lectinic bond, which was reversed by competitive sugars. Binding increased mesangial cell tumor necrosis factor synthesis and decreased prostaglandin E2 production. Several gluten lectinic fractions modulate leukocyte oxidative metabolism, cytotoxicity, and chemotaxis. In IgAGN patients, serum IgA to dietary Ag were sporadically positive and IgAIC containing IgA to dietary components were significantly increased. The affinity of serum IgA to various lectins was increased in some patients. Conversely, no substantial deposition in renal tissue of dietary Ags was observed by immunofluorescence. A gluten-free diet, given to IgAGN patients with high levels of circulating IgAIC and positive antigliadin IgA, was followed by a decrease in the mean levels of both IgAIC and IgA to various dietary Ag, parallel to a reduction in proteinuria. These data suggest that dietary components, such as Ag or lectins, may play a role in IgAGN by promoting IgAIC formation and perhaps favoring mesangial localization via lectinic interactions.