Compare antibody isotypes: IgM, IgG, and IgA in terms of location and primary function.

• A. IgM is abundant in mucosal surfaces; IgG is primarily in blood and tissue fluids.
• B. IgG is the first antibody produced in response.
• C. IgA is responsible for opsonization and complement activation systemically.
• D. IgM is the first antibody produced in response; IgG is systemic with opsonization and complement activation; IgA is secreted at mucosal surfaces.

Answer: (D)

Antibody isotypes specialize by where they act and what they do best. In a primary immune response, the first antibody produced is of the IgM class. IgM is usually found in the blood as a pentamer, making it very effective at activating the classical complement pathway right at the start of an infection. As the response progresses, B cells can undergo class switching to IgG or IgA, among others, to tailor the defense to different sites.

IgG is the major antibody in the circulation and tissues. It handles systemic defense through opsonization, making pathogens easier targets for phagocytes, and by activating complement. It’s also the isotype that provides most of the long-lasting systemic protection and can cross the placenta to protect the fetus.

IgA dominates mucosal surfaces and is secreted in secretions like saliva, tears, and mucus. Secretory IgA is a dimer that helps neutralize pathogens and toxins right at entry points, preventing adhesion and invasion, with limited capacity to trigger inflammation through complement.

So the best description is that IgM is the first antibody produced, IgG operates systemically with opsonization and complement activation, and IgA is secreted at mucosal surfaces to protect those entry points.