Secretory IgA is the prevailung immunoglobulin on the mucous membranes of different tissues. It is a polymer immunoglobulin and in comparison to the serum IgA the secretory IgA has a additional polypeptide chain, the secretory component. The secretory IgA is synthesized locally in the mucosa. Secretory IgA is a very important factor in the immune defence of the mucous membranes. Secretory antibodies are regulated independently from serum antibodies. They show a virus neutralizing effect and activities against bacteria and lifeless noxa. The mechanism is not yet clear. It is possible, that the reaction of the secretory IgA with the antigen prevents settlement of microorganisms on the mucous membranes. The clinical importance of secretory IgA is undoubted especially the deference of the mucosa in cases of a virus infection.