The mucosal tissues and external secretory glands of humans and animals reveal a remarkable preponderance of IgA-producing plasma cells1 which, as demonstrated in animal experiments, are derived from precursors in the organized gut- and bronchus- associated lymphoid tissues (GALT; BALT).2 Despite the general pre-occupation with GALT, several investigators have considered that cells from other sources may contribute to the pool of IgA precursors.3–6 Surgical removal of all visible GALT or thoracic duct drainage in experimental animals results in only a 50% decrease in the intestinal lamina propria plasma cells, implying that alternative sources contribute significantly to the pool of IgA precursor cells. It is plausible that GALT, BALT, and other sources (peritoneum, tonsils, and rectal lymphoid follicles) may preferentially supply IgA-committed, antigen-sensitized cells to restricted (frequently adjacent) mucosal regions.7–10 This possibility has considerable practical impact: inhalation or intranasal immunization might stimulate immune responses preferentially in the upper respiratory and digestive tracts rather than in the genital or lower intestinal secretions, whereas ingestion of antigens or rectal immunization would stimulate responses preferentially in corresponding areas and less, for example, in nasal secretions or tears. Although the induction of antibodies in saliva, tears, and milk by mucosal immunization indicates that different inductive sites overlap in their ability to seed IgA-secreting cells to various effector sites, the extent to which the common mucosal immune system (CMIS) is compartmentalized is not clear. As many pathogens display distinct tropism for certain mucosal sites, exploitation of compartmentalization within the CMIS to direct a immune response to a particular site where the infection can be effectively countered would not only be efficient, but also might avoid undesirable effects elsewhere, and this information may be valuable in the design of vaccines and appropriate immunization routes.