Which immunoglobulin is most abundant in serum and provides long-term protection?

• A. IgM
• B. IgG
• C. IgA
• D. IgE

Answer: (B)

IgG is the antibody most abundant in the blood and the one that provides lasting protection. It accounts for the majority of circulating immunoglobulins and has a relatively long half-life, allowing it to persist in the circulation and keep defending against pathogens over time. IgG is produced during secondary immune responses after affinity maturation, so it has high affinity for its targets. It can neutralize toxins and pathogens, opsonize microbes for enhanced phagocytosis, and activate the classical complement pathway, all contributing to durable immunity. Additionally, IgG crosses the placenta, transferring protection to the fetus and newborn. In contrast, IgM is mainly involved in the initial response and is less abundant later; IgA is key at mucosal surfaces; IgE is involved in allergic responses and defense against parasites.