Question

Which immunoglobulin can cross the placental barrier and protect the new born from infections?

• A. IgG
• B. IgE
• C. IgD
• D. IgA

Hint:

Remember: IgG is the only antibody class that can cross the placenta.
It is transported actively by the FcRn receptor (neonatal Fc receptor), which binds to the constant Fc region of IgG.

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
Maternal antibodies are transferred to the fetus or newborn to provide passive immunity during the critical early stages of life when the offspring's own immune system is immature.
This transfer can occur either transplacentally before birth or via colostrum after birth.

Step 2: Detailed Explanation:
Among the five classes of immunoglobulins, only IgG has the unique ability to cross the placental barrier in humans and certain other mammals (like rodents and primates).
This transplacental transport is not mediated by simple diffusion.
Instead, it is an active, highly selective process facilitated by specific receptors called neonatal Fc receptors (FcRn) located on the syncytiotrophoblast cells of the placenta.
These FcRn receptors bind specifically to the Fc portion of maternal IgG molecules and transport them intact across the placental cell layers into the fetal circulation.
This provides the newborn with a passive immune shield matching the mother's immunological memory.
Other immunoglobulins, such as IgM (too large, pentameric), IgA (monomeric/dimeric), IgE, and IgD, lack compatibility with the FcRn receptor and cannot cross the placenta.
Note: In many domestic animals (such as horses, cattle, and pigs) with epitheliochorial placentation, even IgG cannot cross the placenta, and passive immunity is transferred entirely postnatally via colostrum. However, in general immunology, IgG is the only placental-crossing antibody.

Step 3: Final Answer:
Therefore, the immunoglobulin that crosses the placental barrier is IgG, corresponding to option (A).