This is the dominant immunoglobulin in the healthy mouth.15 SigA is produced by gland-associated immunocytes that are scattered in acini and in clusters adjacent to salivary ducts. SigA is composed of two molecules of heavy and light chains, a secretory component that protects the immunoglobulin from being degraded by proteolytic enzymes, and a J chain. This is a unique joining chain not found in any other immunoglobin that connects the two IgA molecules into a dimeric structure.16
SigA can agglutinate oral bacteria, such as S. mutans, modulate enzyme activity, and inhibit the adherence of bacteria to the buccal epithelium and to enamel.15,17 It does well at interfering with the initial colonization of caries-associated microflora on the tooth surface, but being a salivary protein, it does not always have access to bacteria that are deeper in periodontal pockets. SigA is also a poor activator of the complement system, the biochemical cascade that helps antibodies physically clear pathogens. It is also a poor opsonizer that does not reliably make bacterial cells susceptible to phagocytosis.