Almost a decade ago different isotypes of immunoglobulin were found, by immunohistochemical analysis, to be localized in different plasma cells and it was inferred that circulating IgA was synthesized by the unique class of plasma cells which contained this protein (1,2). Thus it seemed particularly encouraging to efforts to locate the sites of synthesis of secretory IgA (sIgA) when Tomasi and his colleagues found many IgA containing lymphoid cells in the interstitial tissue between acini of human salivary glands (3) and Heremans and coworkers found copious numbers of IgA immunocytes (400,000/mm3) in the lamina propria of human intestine (4). Our analysis of the rabbit ileum by double fluorescent antibody staining confirmed the abundance of IgA lymphoid cells in the lamina propria and revealed the skewed distribution of IgA containing lymphoid cells in favor of intestinal mucosa compared with spleen and peripheral lymph nodes (5). The IgA immunocytes comprise 80–90% of the total population of plasma cells in the rabbit intestinal mucosa but they only make up about 5% of the total in spleen and peripheral lymph nodes. A possible explanation for the preponderance of IgA containing cells in secretory tissue in contrast to their relative scarcity elsewhere is that there are “central” lymphoid tissues which give rise to IgA plasma cell precursors uniquely able to selectively populate gut lamina propria and the interstitia of exocrine glands. Alternatively, each locale may contain a mixed population of lymphocytes which include some that are pre-committed to give rise to IgA-producing daughters and that are selectively stimulated. If, however, lymphocytes are not necessarily committed to the synthesis of a particular Ig class before antigenic stimulation, it may be that the secretory tissue environment can somehow influence plasma cell precursors to generate cells producing IgA.KeywordsPlasma CellLamina PropriaPopliteal Lymph NodePokeweed MitogenHuman Salivary GlandThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.