Which antibody is most commonly found in secretions and mucosal surfaces?

• A. IgG
• B. IgA
• C. IgM
• D. IgE

Answer: (B)

Mucosal immunity relies on a specialized antibody, IgA, especially in its secretory form. IgA is produced by plasma cells in mucosa-associated lymphoid tissue and transported across epithelial cells as a dimer linked by a J chain, then coated with a secretory component. This secretory IgA is the dominant antibody in secretions like saliva, tears, mucus, and along the gastrointestinal and respiratory tracts. It works by neutralizing pathogens and toxins and preventing their attachment to mucosal surfaces, all while minimizing inflammation that could damage delicate mucosal tissues. The secretory component helps protect IgA from proteolysis in these harsh environments, ensuring it remains functional in secretions. Other isotypes—IgG mainly in serum and tissues, IgM as the first responder and often pentameric, and IgE involved in allergies and parasite defense—do not predominate in mucosal secretions. That is why IgA is the antibody most commonly found in secretions and mucosal surfaces.