Final answer:
IgA can cross the epithelial barrier and is found in secretions such as saliva, tears, and breast milk, providing protection to mucosal surfaces.
Step-by-step explanation:
The immunoglobulin that can cross the epithelial barrier is IgA. This antibody exists as a dimeric form in exocrine gland secretions of the mucous membranes, including mucus, saliva, tears, and is also present in breast milk. This form of IgA is responsible for coating and protecting the mucosal surfaces, and it is the only antibody that leaves the interior of the body for this purpose. Secretory IgA binds to pathogens, neutralizing them and preventing infection of the mucosa. Conversely, IgM, which is predominantly found as a pentamer linked by a J chain, is secreted by mucosal plasma cells as well but to a lesser extent and it is not typically associated with crossing epithelial barriers in the same way as IgA.