Stimulation of secretory antibody following oral administration of antigen.

IgA antibodies produced by plasma cells in secretory tissues associated with mucosal surfaces represent a major host defense against infectious agents. Precursors of these IgA-producing cells originate in gut-associated lymphoid tissue and, to a lesser extent, in bronchial-associated lymphoid tissue and migrate via the blood not only to gut but also to other mucosal tissues, forming a common mucosal immune system. Oral administration of antigen leads to priming of precursor cells in the gut and subsequent occurrence of specific secretory IgA (S-IgA) antibody in saliva, milk, tears, and the respiratory and genital tracts. On the basis of a review of the world literature, it has been concluded that oral live vaccines elicit higher antibody titers in remote-site secretions and in serum than do oral killed vaccines, which usually stimulate only relatively low levels of local secretory antibody. Oral immunization induces S-IgA in children and adults (including the elderly) without detectable adverse effects. Despite a number of remaining questions, oral immunization against extraintestinal infections is a promising area for future clinical studies.