The immunoglobulin that crosses placental barrier is
**Core Concept**
The underlying principle being tested is the ability of certain immunoglobulins to cross the placental barrier, providing neonatal immunity. This involves the transfer of **immunoglobulin G (IgG)** from mother to fetus.
**Why the Correct Answer is Right**
IgG is the only immunoglobulin that can cross the placental barrier, providing the fetus with passive immunity against infections. This is due to its structure and the **FcRn receptor** on placental cells, which facilitates the transfer of IgG from the mother's bloodstream to the fetus.
**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because IgA is primarily involved in mucosal immunity and does not cross the placental barrier.
**Option B:** Incorrect as IgM is the first immunoglobulin produced in response to an infection and is too large to cross the placental barrier.
**Clinical Pearl / High-Yield Fact**
It's crucial to remember that the ability of IgG to cross the placental barrier is vital for protecting the newborn against infections until its own immune system matures.
**Correct Answer:** D. IgG