Which immunoglobulin listed is most abundant in the bloodstream?

• A. IgG
• B. IgA
• C. IgD
• D. IgE

Answer: (A)

The key idea is that different immunoglobulins specialize in different body compartments, and one class dominates the bloodstream. IgG is the most abundant antibody in serum, making up the majority of circulating antibodies. This high level reflects its long half-life and versatile role in systemic immunity: it neutralizes toxins and viruses, promotes phagocytosis through opsonization, and activates the classical complement pathway. IgA is mainly found in mucosal areas and secretions, with only a portion in the blood. IgM is prominent early in an immune response and is large, often circulating during acute stages but not as plentiful in steady-state serum. IgD is present in very small amounts and mainly acts as a B cell receptor. Therefore, IgG best fits the description of the most abundant antibody in the bloodstream.