Final answer:
The correct answer is IgA is secreted into the bloodstream in its monomeric form, while dimeric IgA is secreted into various mucosal secretions like saliva and breast milk. Dimeric IgA serves to protect mucosal surfaces by trapping pathogens.
Step-by-step explanation:
IgA is secreted into the bloodstream in its monomeric form, whereas dimeric IgA is secreted into mucus such as gastrointestinal fluid, colostrum, saliva, tears, and sweat. The dimeric IgA, which consists of two IgA monomers connected by a J chain, is predominantly found in mucosal secretions and plays a crucial role in protecting mucosal surfaces by trapping pathogens. In contrast, monomeric IgA is present in the serum at lower levels.
The question compares IgA with another immunoglobulin, IgM, which is also secreted into bodily fluids but as a pentameric structure with five monomers joined together by a J chain. While IgA is primarily important for mucosal immunity, IgM is the first antibody produced and secreted by B cells during the primary immune response and is valuable in diagnosing active or recent infections.