Secretory IgA (SIgA), which is one of the humoral factors responsible for the immune protection of large areas of mucosal surfaces, is produced by plasma cells resident in the submucosae and glandular stroma. However, specific induction of SIgA antibodies occurs largely through the common mucosal immune system, whereby antigens presented to the mucosa-associated lymphoid tissues, such as intestinal Peyer's patches, stimulate B lymphocytes committed to IgA synthesis. These cells enter the circulation via lymphatics and finally home to several remote secretory tissues. The regulatory cells and factors that govern the mucosal immune response are beginning to be elucidated. Several bacterial and viral antigens have been used to evoke SIgA antibodies experimentally in humans, but only a few oral vaccines have been developed for clinical application. Considerable scope exists for the identification of appropriate immunogens and their formulation in delivery vehicles with suitable adjuvants to induce protective antibodies against a variety of pathogens that invade through mucosal surfaces. Several mucosal pathogens, however, secrete proteases that specifically cleave human IgA1. The full significance of this evasion mechanism and the ways of circumventing it must be addressed to maximize the effectiveness of mucosal immunity.