Which Ig crosses through placenta: September 2008
**Core Concept**
The placenta plays a crucial role in the transfer of immunoglobulins (Ig) from mother to fetus, influencing fetal immune development. One specific Ig has been identified to cross the placenta, contributing to the fetal immune system.
**Why the Correct Answer is Right**
IgG is the only immunoglobulin that can cross the placenta. This occurs through a process called receptor-mediated transcytosis, facilitated by the neonatal Fc receptor (FcRn) on the placental syncytiotrophoblast. FcRn binds to the Fc region of IgG, allowing it to be transported across the placenta to the fetus. This transfer of IgG from mother to fetus provides the newborn with passive immunity, protecting against infections for the first few months of life.
**Why Each Wrong Option is Incorrect**
* **Option A:** IgA is primarily found in mucosal secretions and does not cross the placenta. It plays a crucial role in mucosal immunity but is not transferred from mother to fetus.
* **Option B:** IgM is the first immunoglobulin produced in response to an infection and is not able to cross the placenta. It is also the largest immunoglobulin and is primarily found in the serum.
* **Option C:** IgE is involved in allergic reactions and is not transferred from mother to fetus. It plays a key role in the body's response to parasites and in allergic diseases.
**Clinical Pearl / High-Yield Fact**
IgG antibodies transferred from mother to fetus provide essential protection against infections, such as rubella and tetanus, during the first few months of life. This emphasizes the importance of maternal vaccination and the need for pregnant women to receive necessary vaccinations.
**Correct Answer: D. IgG.**