Question

Which one of the following Immunoglobulins is transmitted to the fetus through the placenta?

• A. IgG
• B. IgA
• C. IgM
• D. IgD

Hint:

Only the smallest immunoglobulin class crosses the placenta using an active receptor system.

Answer 1

The Correct Option is A

A newborn gets ready made protection from its mother, and this question asks which antibody makes that journey across the placenta before birth.

1. IgG: This is the smallest of the antibody classes, and the placenta has a dedicated receptor that actively pulls it across from mother to fetus. This is exactly why newborns carry maternal antibodies against diseases the mother has already met.
2. IgA: This antibody protects mucosal surfaces like the gut lining, and the infant gets it mainly from breast milk and colostrum after birth, not from the placenta.
3. IgM: This is the biggest antibody molecule of the group, and its size keeps it from passing through the placenta at all. If a baby is born with high IgM, doctors take it as a sign the fetus itself made antibodies against an infection.
4. IgD: This one sits on B cells as a receptor and circulates in only tiny amounts in blood, so it plays no real role in fetal protection.

Placental transfer needs both a small enough antibody and a matching receptor system, and IgG is the only class that meets both conditions.

So the immunoglobulin transmitted to the fetus through the placenta is IgG.