Which antibody is most effective at systemic pathogen neutralization and crosses the placenta?

• A. IgG
• B. IgA
• C. IgM
• D. IgD

Answer: (A)

IgG provides systemic pathogen neutralization and can cross the placenta. It circulates in high amounts in the blood as a monomer, allowing it to diffuse through tissues, bind a wide range of pathogens, neutralize toxins, and opsonize microbes for phagocytosis. It also efficiently activates the classical complement pathway, enhancing bacterial clearance. Importantly, IgG is transported across the placenta via the neonatal Fc receptor (FcRn), delivering protective antibodies to the fetus. In contrast, IgA mainly protects mucosal surfaces as secreted dimers and does not cross the placenta; IgM is a large pentamer that is great at activating complement but largely remains in the bloodstream and also does not cross the placenta; IgD has limited secreted antibody and functions primarily as a B cell receptor.