Final answer:
Immunoglobulin A (IgA) is the antibody most commonly associated with mucous membranes and secretions, playing a key role in the body's first line of defense against pathogens at mucosal surfaces.
Step-by-step explanation:
The class of antibody most commonly associated with mucous membranes and secretions is Immunoglobulin A (IgA). IgA is critical for mucosal immunity and is found in a variety of secretions including tears, saliva, breast milk, and secretions of the gastrointestinal and respiratory tracts. Dimeric IgA is able to trap pathogens in mucus to prevent infections, making it an important first line of defense in the immune system.