Which antibody type is predominant in the serum of neonates born after full-term gestation?

• A. Infant IgA
• B. Infant IgG
• C. Infant IgM
• D. Maternal IgG

Answer: (C)

IgG is actively transferred from mother to fetus across the placenta, providing the newborn with passive immunity. In a healthy term neonate, this placental transfer results in maternal IgG becoming the dominant antibody in the infant’s serum. IgM and IgA do not cross the placenta in significant amounts, and the infant’s own production of IgG begins after birth, so the initial serum IgG is primarily maternal. A detectable IgM at birth would suggest an infection or immune activation, which is not typical for a healthy term newborn. Therefore, maternal IgG is the predominant antibody in the serum of term neonates.