SIgA

Secretory immunoglobulin A (sIgA) is the principal immunoglobulin class in mucosal secretions, including saliva, tears, mucus, bile, and breast milk. It most often appears as a dimer of IgA molecules linked by a J chain and associated with a secretory component derived from the polymeric Ig receptor (pIgR). In humans, IgA has two subclasses, IgA1 and IgA2, which can be present in mucosal secretions. The secretory component enhances stability in harsh mucosal environments.

Production and transport: sIgA is produced by plasma cells in mucosa-associated lymphoid tissue such as Peyer’s patches and tonsils. Dimeric IgA binds to pIgR on the basolateral surface of mucosal epithelial cells and is transcytosed to the lumen. Proteolytic cleavage releases the secretory component, leaving secretory IgA attached to it. This configuration protects IgA from proteolysis and facilitates its persistence in secretions.

Functions: sIgA mediates immune exclusion by neutralizing pathogens and toxins and by preventing microbial adhesion to epithelial surfaces. It can trap antigens in mucus and promote their removal with little inflammation, contributing to tolerance toward commensal bacteria. It may participate in antigen sampling and modulate mucosal immune responses; the secretory component also confers proteolytic resistance.

Distribution and clinical notes: sIgA is abundant in secretions but scarce in serum. Reduced mucosal sIgA is linked to increased susceptibility to infections; selective IgA deficiency is the most common primary immunodeficiency. Levels vary with age, infection, and mucosal inflammation.