Final answer:
Mucosal surfaces are protected by dimeric IgA, which is secreted in bodily fluids like mucus and saliva, and prevents pathogen attachment. The correct option is a.
Step-by-step explanation:
The mucosal epithelia of the GI tract, the respiratory, urinary, and genital tracts are protected by dimeric IgA. IgA exists in two forms; the four-chain monomer found in the blood and the eight-chain dimer that is secreted into mucus, saliva, tears, and breast milk.
Dimeric IgA is crucial for mucosal immunity because it can neutralize pathogens by preventing their attachment to mucosal surfaces. Secretory IgA plays an important role by trapping pathogens in mucus to be eliminated from the body.The correct option is a.