In mucosal secretions, which antibody is most abundant and typically secreted as a dimer?

• A. IgG
• B. IgM
• C. IgE
• D. IgA

Answer: (D)

Secretory IgA is the dominant antibody in mucosal secretions, and it is typically secreted as a dimer. It is produced by plasma cells in mucosa-associated lymphoid tissue and exists in secretions as two IgA monomers linked by a J chain. Transport across the mucosal epithelium is mediated by the polymeric immunoglobulin receptor; when released into the lumen, the receptor’s secretory component remains attached to IgA, shielding it from proteases. This dimeric, secretory form—secretory IgA—specializes in neutralizing pathogens and toxins and preventing their attachment to mucosal surfaces (immune exclusion). In contrast, IgG is the main serum antibody, IgM is produced early as a pentamer, and IgE is involved in allergy and parasitic defenses, not mucosal secretions.