Which immunoglobulin is primarily responsible for mucosal immunity and is produced as a dim er?

• A. IgA
• B. IgG
• C. IgM
• D. IgE

Answer: (A)

Secretory IgA is the antibody primarily responsible for mucosal immunity, and it is produced as a dimer. Plasma cells in mucosal tissues synthesize two IgA molecules linked by a J chain to form a dimer, which is then transported across the epithelium by the polymeric Ig receptor. In secretions, IgA acquires a secretory component that protects it from proteolysis, allowing it to function effectively in the harsh environments of the gut, respiratory tract, and other mucosal surfaces. This form of IgA neutralizes pathogens and toxins and helps prevent their attachment to mucosal cells, providing a first line of defense without provoking strong inflammation. By contrast, IgG is the main systemic antibody, IgM is a pentamer that appears early in responses, and IgE is involved in allergic reactions and parasite defense, not mucosal immunity.