A generalized secretory IgA response can be induced by ingestion of various antigens due to dissemination of sensitized precursors of IgA plasma cells from gut-associated lymphoid tissue to various secretory glands. Oral ingestion of a bacterial antigen by volunteers led to the parallel appearance of secretory IgA antibodies in several external secretions that was preceded by a transitory detection of IgA antibody-secreting cells in the peripheral blood. Naturally occurring secretory and serum IgA antibodies as well as secretory and serum antibodies induced by mucosal immunization belong predominantly to the IgA1 subclass; however, in external secretions IgA2 is the predominant subclass of natural antibodies to endotoxin and lipoteichoic acid. Although in external secretions specific IgA antibodies are of polymeric form, serum IgA antibodies to different antigens display considerable variability with respect to their distribution in polymeric and monomeric forms. However, after experimental infection, serum IgA antibodies to the influenza virus hemagglutinin are almost exclusively polymeric.