Question:
Which one of the following Immunoglobulins is transmitted to the fetus through the placenta?
Which one of the following Immunoglobulins is transmitted to the fetus through the placenta?
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Only the smallest immunoglobulin class crosses the placenta using an active receptor system.
Updated On: Jul 7, 2026
- IgG
- IgA
- IgM
- IgD
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The Correct Option is A
Solution and Explanation
Step 1: Understanding the Concept.
A newborn is not able to make its own antibodies in useful amounts right after birth, so it depends on antibodies passed from the mother. This passive protection can come either across the placenta before birth or through breast milk after birth. The question asks which class of antibody crosses the placenta.
Step 2: Key Formula or Approach.
Whether an immunoglobulin crosses the placenta depends mainly on its size and on whether the placenta has a receptor for it. Small antibodies that fit through placental transport channels can cross, while large or structurally different ones cannot.
Step 3: Detailed Explanation.
IgG is the smallest of the major immunoglobulin classes and is actively carried across the placenta using a specific receptor on the placental cells. This gives the newborn passive immunity against many infections the mother has already been exposed to, and this protection lasts for the first few months of life.
IgA is important in mucosal surfaces and is passed to the infant mainly through breast milk, especially colostrum, not through the placenta.
IgM is the largest immunoglobulin and its size prevents it from crossing the placenta. In fact, if a newborn has raised IgM at birth, it points to an infection the fetus itself fought, since maternal IgM cannot have crossed over.
IgD is present in very small amounts in serum and mainly acts as a receptor on B cells rather than being transferred to the fetus.
Step 4: Final Answer.
The immunoglobulin transmitted to the fetus through the placenta is IgG.
A newborn is not able to make its own antibodies in useful amounts right after birth, so it depends on antibodies passed from the mother. This passive protection can come either across the placenta before birth or through breast milk after birth. The question asks which class of antibody crosses the placenta.
Step 2: Key Formula or Approach.
Whether an immunoglobulin crosses the placenta depends mainly on its size and on whether the placenta has a receptor for it. Small antibodies that fit through placental transport channels can cross, while large or structurally different ones cannot.
Step 3: Detailed Explanation.
IgG is the smallest of the major immunoglobulin classes and is actively carried across the placenta using a specific receptor on the placental cells. This gives the newborn passive immunity against many infections the mother has already been exposed to, and this protection lasts for the first few months of life.
IgA is important in mucosal surfaces and is passed to the infant mainly through breast milk, especially colostrum, not through the placenta.
IgM is the largest immunoglobulin and its size prevents it from crossing the placenta. In fact, if a newborn has raised IgM at birth, it points to an infection the fetus itself fought, since maternal IgM cannot have crossed over.
IgD is present in very small amounts in serum and mainly acts as a receptor on B cells rather than being transferred to the fetus.
Step 4: Final Answer.
The immunoglobulin transmitted to the fetus through the placenta is IgG.
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