Which antibody is found in secretions and breast milk?

• A. IgG
• B. IgA
• C. IgM
• D. IgE

Answer: (B)

Secretory IgA is the antibody found in mucosal secretions and breast milk. It is produced as a dimer linked by a J chain and transported across mucosal epithelial cells by the polymeric Ig receptor, ending up with a secretory component attached. This secretory piece protects IgA from proteolysis and helps it function in the gut, airways, and other surfaces by neutralizing pathogens before they invade. That’s why secretions like saliva, tears, mucous, and breast milk are rich in IgA, providing passive protection to nursing infants. Other isotypes have different roles: IgG circulates in blood and crosses the placenta, IgM is the first antibody produced in a new infection, and IgE is involved in allergic responses and defense against parasites.