Immunology — Which class of human immunoglobulin (antibody) can cross the placenta to provide passive immunity to the fetus?

Introduction / Context:
Maternal antibodies protect the developing fetus and newborn before their own immune system fully matures. Understanding which immunoglobulin is transported across the placenta is vital in clinical immunology, vaccinology, and perinatal medicine.

Given Data / Assumptions:

• Placental transfer occurs via receptor-mediated transport.
• Different immunoglobulin classes vary in structure and transport properties.
• The focus is on normal human pregnancy.

Concept / Approach:
IgG is the only immunoglobulin class that efficiently crosses the human placenta. This occurs through neonatal Fc receptors (FcRn) on placental syncytiotrophoblasts, which bind the Fc region of IgG and transport it to the fetal circulation. IgM, being pentameric and large, does not cross the placenta. IgA (mostly dimeric in secretions) is important for mucosal immunity and is transferred in breast milk, not across the placenta. IgD has limited known roles in serum and does not undergo significant placental transfer.

Step-by-Step Solution:

Verification / Alternative check:
Cord blood serology demonstrates IgG specificities reflecting maternal exposure or vaccination, supporting placental transfer of IgG.

Why Other Options Are Wrong:

• IgM: too large; does not cross the placenta.
• IgA: transferred mainly via breast milk (secretory IgA), not placentally.
• IgD: not known for placental transfer.

Common Pitfalls:
Assuming all antibodies cross equally; structure and Fc receptor interactions determine transfer efficiency.

Final Answer:
IgG