Secretory IgA at mucosal surfaces is typically present as which form?

• A. Monomeric IgA
• B. IgA bound to IgG
• C. Dimeric secretory IgA
• D. Pentameric IgA

Answer: (C)

Secretory IgA at mucosal surfaces is produced as a dimeric molecule that includes a joining (J) chain and, after transport across epithelial cells, a secretory component attached. Two IgA monomers linked by the J chain are bound by the polymeric Ig receptor on the basolateral surface, transported through the cell, and released into secretions with the secretory component still bound. This secretory IgA is specifically adapted for the harsh mucosal environment: the secretory component helps protect it from proteolysis and aids in stabilizing the molecule so it can effectively neutralize pathogens and toxins at entry points. Monomeric IgA is more typical in serum, while IgM is pentameric and not the mucosal IgA form; IgA bound to IgG is not the standard secretory IgA form.