Immunity: placental transfer of antibody: practice question

A Physiology question in the style of the NEET PG paper. Answer it first, then read why the right option is right and why each of the others is wrong.

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Physiology · Immunity: placental transfer of antibody

Sample item

A Rh-negative mother who did not receive anti-D prophylaxis after her first Rh-positive baby has a second Rh-positive fetus with hydrops. Which antibody class mediates the haemolysis?

Original practice item, not a recalled exam question. Nothing here is recorded.

The answer and the reasoning

B. Maternal IgG, which crosses the placenta by the neonatal Fc receptor

Sensitisation at the first delivery produces anti-D antibodies. Subsequent exposure boosts IgG, the only immunoglobulin class that is transported across the syncytiotrophoblast by FcRn. It coats fetal D-positive red cells, which are destroyed in the spleen. IgM is a pentamer that is too large to cross.

Option by option

  • A. Fetal IgE, which is made against paternal antigen

    IgE is not involved and is not produced against red cell antigens in this way.

  • B. Maternal IgG, which crosses the placenta by the neonatal Fc receptor (correct)

    Correct. IgG is actively transported across the placenta and causes extravascular haemolysis.

  • C. Maternal IgM, which crosses the placenta by pinocytosis

    IgM is too large to cross the placenta, though it is the first antibody made after sensitisation.

  • D. Maternal IgA from colostrum, which is absorbed in utero

    IgA is secreted in milk after birth and does not cross the placenta.

Source note: Original NextPrep sample item — pending expert review. This item is original practice material awaiting independent expert review, so confirm important facts in a standard textbook.

More Physiology questions

Questions about this Physiology question

What is the answer to this Physiology question?

The best answer is B, Maternal IgG, which crosses the placenta by the neonatal Fc receptor. Sensitisation at the first delivery produces anti-D antibodies.

Why is "Fetal IgE, which is made against paternal antigen" not the answer?

IgE is not involved and is not produced against red cell antigens in this way.

Why is "Maternal IgM, which crosses the placenta by pinocytosis" not the answer?

IgM is too large to cross the placenta, though it is the first antibody made after sensitisation.

Why is "Maternal IgA from colostrum, which is absorbed in utero" not the answer?

IgA is secreted in milk after birth and does not cross the placenta.

What concept does this question test?

It tests Immunity: placental transfer of antibody in Physiology. Subsequent exposure boosts IgG, the only immunoglobulin class that is transported across the syncytiotrophoblast by FcRn. It coats fetal D-positive red cells, which are destroyed in the spleen. IgM is a pentamer that is too large to cross.

How hard is this question?

We tag it as an application question: it asks you to use a fact on a patient or a finding. The tag is our own editorial judgement, not an official difficulty rating, so read the reason for every option, not only the answer.

How should I study Immunity: placental transfer of antibody?

For each system, sketch the key graph (pressure–volume loop, oxygen dissociation curve, nephron handling) and explain it aloud. Then use the flashcards and the other Physiology questions on this site to test what stuck.

Is this a real NEET PG question?

No. It is an original practice item (editorial status: sample, version 1). NextPrep does not publish recalled or leaked questions, and independent faculty review of this item has not been completed yet, so confirm important facts in a standard textbook.