Mucosal immunity is primarily mediated by which immunoglobulin class, typically present as a dimeric secretory form?

• A. IgG
• B. IgA
• C. IgM
• D. IgE

Answer: (B)

Mucosal immunity is carried primarily by secretory IgA, which is produced as a dimer linked by a J chain and then transported across mucosal epithelial cells where it acquires a secretory component. This dimeric secretory form is specially suited to the harsh, protease-rich mucosal environment and provides immune exclusion: it neutralizes pathogens and toxins before they attach to or invade the epithelium, reducing inflammation and tissue damage. While other immunoglobulins play important roles elsewhere—IgG with systemic functions and some presence in secretions, IgM often as a pentamer in early responses, and IgE in parasite defense and allergies—secretory IgA is the dominant antibody at mucosal surfaces and is the primary mediator of mucosal immunity in its dimeric secretory form.