Which antibody isotype provides most effective opsonization and activates complement via the classical pathway?

• A. IgG
• B. IgA
• C. IgM
• D. IgE

Answer: (A)

Opsonization and classical complement activation primarily rely on how antibodies interact with immune cells and complement proteins. IgG is the antibody isotype that coats pathogens in a way that efficiently engages Fc gamma receptors on phagocytes, promoting internalization and destruction of the invader. At the same time, when bound to antigen, IgG can initiate the classical complement pathway by allowing C1q to bind to its Fc regions, leading to complement activation.

While IgM is a very potent activator of the classical pathway because of its pentameric structure, it is not as effective at opsonizing pathogens via Fc receptors, and IgA and IgE do not provide strong opsonization through Fc receptor–mediated phagocytosis in this context. Therefore, IgG best fits both delivering efficient opsonization and activating the classical complement pathway.