Question
Secretory IgA
Answer
Secretory IgA (sIgA) is the predominant immunoglobulin class present in external body secretions — saliva, tears, colostrum/breast milk, and mucosal secretions of the respiratory, gastrointestinal, and genitourinary tracts — forming the first line of humoral defence at mucosal surfaces.
Structure: unlike serum IgA (predominantly monomeric), secretory IgA exists as a dimer of two IgA monomers joined by a J (joining) chain, plus an additional secretory component (SC) — a fragment of the poly-Ig receptor acquired during transport across mucosal epithelial cells, which protects the molecule from proteolytic degradation by enzymes present in secretions (e.g., gut proteases).
Production and transport: synthesized by plasma cells in the lamina propria beneath mucosal epithelium; the dimeric IgA-J chain complex binds the polymeric immunoglobulin receptor (pIgR) on the basolateral surface of the overlying epithelial cell, is transcytosed across the cell, and released at the apical (luminal) surface with the receptor fragment (secretory component) still attached.
Function: acts mainly by immune exclusion — it binds and agglutinates microorganisms and toxins at the mucosal surface, preventing their adherence to and invasion through the epithelium (it does not efficiently activate complement or opsonize, unlike IgG); also neutralizes viruses and toxins within the mucus layer.
Clinical significance: provides passive mucosal protection to breastfed infants via colostrum (particularly important in the first days after birth, when colostrum is especially IgA-rich); selective IgA deficiency is the most common primary immunodeficiency, predisposing to recurrent sinopulmonary and gastrointestinal infections, and (in some cases) increased risk of atopic and autoimmune disease.