Immunoglobulin A (IgA) is a dominant immunoglobulin of the mucosal surfaces, but it is also present in plasma. In men and in hominoid primates it occurs in two subclasses: IgA1 and IgA2. Circulating IgA is mostly IgA1 monomer, secretory IgA is mostly dimer or tetramer with varying content of IgA1 and IgA2 on individual mucosal surfaces. Its main physiological function is a defence of the mucosal surfaces against infection. It binds either specifically to bacterial antigens or through its O-linked glycosidic chains, it binds to the lectins of bacterial cells and thus protects mucosal surfaces against bacterial adhesion and infection. On each of its heavy chain, IgA1 has at least two N-glycosidically bound oligosaccharides and 3 to 5 O-linked side-chains. The occurrence of O-glycosidically bound glycans on other circulating immunoglobulins is rare. An aberrant composition of these glycans may be an antigenic determinant for naturally occurring circulating antibodies. The resulting IgA-containing immune complexes, which are deposited in the glomeruli, may be the cause of IgA nephropathy. IgA glomerular deposits are also frequently present in many other primary and systemic glomerulonephritides.