Final answer:
The mucosal epithelia of various body tracts are protected by dimeric IgA, which is found in bodily secretions and can trap and neutralize pathogens. It is the only antibody to leave the interior of the body to protect body surfaces.
Step-by-step explanation:
The mucosal epithelia of the gastrointestinal tract, respiratory, urinary, and genital tracts, and the mammary glands are protected by dimeric IgA. IgA exists in two forms, a four-chain monomer in the blood and an eight-chain structure, or dimer, in exocrine gland secretions of the mucous membranes. Dimeric IgA is the only antibody that leaves the interior of the body to protect body surfaces, and it is found in bodily secretions such as mucus, saliva, tears, breast milk, and secretions of the gastrointestinal and respiratory tracts. The dimeric IgA traps pathogens in mucus and neutralizes them, preventing infection of the mucosa.