Final answer:
The secreted form of IgA includes a J chain, which serves to facilitate polymerization and secretion of dimeric IgA, enhancing protection at mucosal surfaces.
Step-by-step explanation:
The secreted form of IgA includes a J chain. The purpose of the J chain is to facilitate polymerization and secretion of IgA, particularly its dimeric form in exocrine gland secretions like mucus, saliva, and tears. This J chain, a 15 Kd polypeptide, joins IgA monomers via disulfide bonds to the tailpieces, resulting in a stable multimeric structure. This multimerization enhances the effectiveness of IgA in neutralizing pathogens and protecting mucosal surfaces in the body.