The available information concerning the origin of IgA-producing plasma cells and the spectrum of IgA-associated antibodies found in external secretions provide arguments that support two pathways of stimulation for a secretory humoral immune response. In addition to an explicitly local immune response induced by a topical antigen application, a second mechanism of induction operating through the sensitization of GALT, and possibly BALT, emerges. The latter pathway of stimulation leads to the appearance of specific IgA-associated antibodies in secretions of mammary, salivary, and lacrymal glands (and perhaps other sites), all of which suggests the existence of a common mucosal secretory system. Not yet explained are the mechanisms involved in the homing to secretory glands of cells sensitized in the remote lymphoid tissues (GALT and BALT), the differentiation patterns of these cells, and the regulation of selective IgA expression.