Final answer:
IgA is the immunoglobulin that guards mucosal surfaces against foreign organisms by being present in secretions like mucus and breast milk and trapping or neutralizing pathogens.
Step-by-step explanation:
The immunoglobulin that protects body surfaces such as mucosal surfaces from foreign microorganisms is IgA. This antibody is present in various secretions like mucus, saliva, and tears, as well as in mother's breast milk which is crucial for protecting infants from disease. It exists in two forms: a four-chain monomer in blood and an eight-chain structure, or dimer, in the secretions of the mucous membranes, making it uniquely adapted to protect these areas. Both neutralization of pathogens and trapping them in mucus are key functions of secretory IgA, thereby preventing infections at mucosal surfaces.