Final answer:
The antibody isotype predominantly secreted in mucosal tissues is IgA, which is critical in protecting mucous membranes by neutralizing pathogens and preventing them from adhering to mucosal surfaces.
Step-by-step explanation:
The antibody isotype that is predominantly secreted in mucosal tissues is IgA. This antibody plays a critical role in the immune protection of mucous membranes, which are exposed to the external environment. These include the mucosa of the gastrointestinal, respiratory, and genitourinary tracts. IgA exists as both a serum antibody in monomeric form and a secretory antibody in dimeric form. In its dimeric form, IgA is equipped with a secretory component that enables it to be transported across mucosal epithelia and be present in secretions such as saliva, tears, breast milk, and mucus.
The presence of IgA in various secretions such as breast milk confers passive immunity to infants, offering protection against pathogens. IgA antibodies operate by binding to pathogens and neutralizing them directly or preventing them from adhering to and penetrating the mucosal surfaces, therefore playing a crucial role in the local humoral immune response and protecting from infections.