Final answer:
The poly-Ig receptor binds to dimeric IgA by the J chain, which facilitates IgA's role in protecting mucosal surfaces.
Step-by-step explanation:
The poly-Ig receptor on the basolateral surface of epithelial cells binds to dimeric IgA by the J chain. IgA exists in two primary forms, the monomer that is found in the serum and the dimer which is present in secretions across mucosal surfaces such as saliva, mucus, tears, and breast milk.
The dimeric IgA is linked by a J chain that is essential for its polymeric structure and is secreted into bodily fluids to play a critical role in mucosal immunity.