Immunoglobulin A is the predominant antibody associated with mucosal surfaces, and it is produced in amounts that exceed those of all serum antibodies. Traditionally, the major role of secretory IgA was thought to be to bind foreign antigens and thereby prevent their entry into the body. This continues to be envisioned as the major host-defense function of IgA. Recently, however, it has been suggested that IgA also has the important ability to neutralize viruses intracellularly. It has also been suggested that IgA is capable of excreting antigens as immune complexes through the epithelium and back into the lumen of the gastrointestinal tract.