Secretory immunoglobulin A (sIgA) is the principal mediator of specific immunity on human mucosal surfaces. Although the exact mechanisms are partly unclear, it is generally accepted that sIgA antibodies maintain the integrity of the mucous membranes by reducing their colonization by microorganisms, by neutralizing toxins and viruses, and by preventing the penetration of microbial and other antigens and allergens through the surfaces.'IgA in serum appears to play an important role in the regulation of inflammatory responses particularly in submucosal tissues.'Recent studies in rodents have suggested, furthermore, that serum IgA is involved in the disposal of antigens from the circulation.'For sIgA to successfully protect mucosal surfaces it must resist proteolytic attack from digestive and microbial enzymes that occur in these environments. Changes in the structure of IgA induced by bacteria was first demonstrated in vitro by Miiller in 1971.3 He reported that strains of Neisseria meningitidis and N. gonorrhoeae possess proteolytic activity that invariably induce selective changes in the electrophoretic mobility of human IgA. Although the exact nature of these changes were not identified, Milller suggested that bacterium-induced alterations of IgA is a possible factor in the pathogenesis of infections caused by N. meningitidis and N. gonorrhoeae. Strains of a number of other bacterial species, including Pseudomonas aeruginosa, Bacteroides melaninogenicus, and Proteus vulgaris, were shown to have a strong proteolytic activity capable of causing progressive degradation of IgA. In addition, several human-pathogenic organisms induced changes in the electrophoretic mobility of IgA and various other human serum glycoproteins by attacking their carbohydrate moieties. The existence of a bacterial proteolytic enzyme with the capacity to induce specific cleavage of IgA in the hinge region, yielding intact Fab, and Fc, fragments was first reported by Metha and coworkers in 1973.4 The enzyme was demonstrated in human feces of six normal subjects examined. The intestinal bacterium responsible for this cleavage has never been identified. Subsequent studies by Plaut, Genco, and Tomasi revealed that a similar enzymatic activity is present in human saliva, and that an organism indigenous to the oral cavity, Streptococcus sanguis, produces an enzyme with this characteristic activity.'