Which antibody is most abundant in blood serum?

• A. IgM
• B. IgG
• C. IgA
• D. IgE

Answer: (B)

IgG is the most abundant antibody in blood serum. It makes up the majority of circulating immunoglobulins because, after initial IgM responses, B cells switch to IgG and produce large quantities with high affinity for antigens. This high abundance allows IgG to perform key systemic defense roles: neutralizing pathogens and toxins, tagging microbes for destruction (opsonization), and triggering the classical complement pathway. IgG also has a relatively long half-life in serum and can cross the placenta, giving newborns passive immunity. Other antibodies play important roles too, but IgM is mainly produced early in infection and is less abundant in steady-state serum, IgA is prioritized for mucosal secretions, and IgE exists at very low levels, focusing on allergic responses and parasite defense.