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Secretory IgA (sIgA) consists of two IgA monomers joined by the J-chain and furthermore a secretory component. It is secreted in plasma cells based in the lamina propia of mucosal membranes. Synthesis of sIgA is independent from the synthesis of serum IgA. This means lack of serum IgA does not necessarily mean a lack of sIgA. Secretory IgA is the major immunoglobulin in saliva, tears, colostrum, nasal mucous, mother´s milk, tracheobronchial and gastrointestinal secretes. It plays a major role in preventing adherence of microorganisms to mucosal sites, in activation of the alternative complement pathway and in activating inflammatory reactions. Newborns are provided with sIgA by mother´s milk and are passively immunized against gastrointestinal infections.