Question

Which one of the following is the only immunoglobulin to cross the placenta?

• A. IgM
• B. IgG
• C. IgA
• D. IgD

Answer 1

The Correct Option is B

Immunoglobulins (antibodies) play a crucial role in the immune system, providing defense against pathogens. Among them, IgG is the only antibody that crosses the placenta, providing passive immunity to the fetus. 

Step 1: Understanding Placental Transfer of Antibodies - The placenta serves as a barrier but allows certain molecules, like IgG, to pass through to provide passive immunity to the fetus. - This transfer occurs through the neonatal Fc receptor (FcRn), which binds to maternal IgG and transports it across the placenta. 

Step 2: Evaluating the Options - Option (A) - Incorrect: IgM is the first antibody produced in response to infection but does not cross the placenta due to its large pentameric structure. - Option (B) - Correct: IgG is the only immunoglobulin that crosses the placenta, providing passive immunity to the fetus. - Option (C) - Incorrect: IgA is mainly found in mucosal secretions (tears, saliva, and breast milk) but does not cross the placenta. - Option (D) - Incorrect: IgD is primarily involved in B-cell activation and does not cross the placenta. 

Step 3: Conclusion Since IgG is the only antibody that crosses the placenta and provides immunity to the fetus, the correct answer is option (B).