Hypersensitivity: type II haemolysis: practice question

A Pathology 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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Pathology · Hypersensitivity: type II haemolysis

Sample item

A 22-year-old woman recovering from Mycoplasma pneumonia has pallor, jaundice and bluish discolouration of the fingers and ears in cold weather. The direct antiglobulin test is positive with anti-C3 only and negative with anti-IgG. Which antibody explains the haemolysis?

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

The answer and the reasoning

D. IgM autoantibody against the I antigen, binding red cells at low temperature

Cold agglutinin disease is a type II hypersensitivity in which IgM antibodies, commonly anti-I after Mycoplasma infection, bind red cells in the cooler peripheral circulation and fix complement. On returning to warmer areas the IgM detaches, leaving C3 on the cells, so the direct antiglobulin test is positive for C3 alone. Haemolysis is mainly extravascular and agglutination causes acrocyanosis.

Option by option

  • A. IgG autoantibody against Rh antigens, binding red cells at 37 degrees Celsius

    Warm IgG autoantibodies give an IgG-positive direct antiglobulin test and no cold-induced acrocyanosis.

  • B. Biphasic IgG antibody against the P antigen (Donath-Landsteiner)

    The Donath-Landsteiner antibody causes paroxysmal cold haemoglobinuria with a biphasic IgG pattern, usually after viral illness in children.

  • C. IgE antibody against red cell membrane proteins

    IgE antibodies mediate type I reactions and do not cause haemolysis with C3 coating.

  • D. IgM autoantibody against the I antigen, binding red cells at low temperature (correct)

    Correct. Cold-reactive IgM against I antigen with C3-only coating fits the picture.

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.

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Questions about this Pathology question

What is the answer to this Pathology question?

The best answer is D, IgM autoantibody against the I antigen, binding red cells at low temperature. Cold agglutinin disease is a type II hypersensitivity in which IgM antibodies, commonly anti-I after Mycoplasma infection, bind red cells in the cooler peripheral circulation and fix complement.

Why is "IgG autoantibody against Rh antigens, binding red cells at 37 degrees…" not the answer?

Warm IgG autoantibodies give an IgG-positive direct antiglobulin test and no cold-induced acrocyanosis.

Why is "Biphasic IgG antibody against the P antigen (Donath-Landsteiner)" not the answer?

The Donath-Landsteiner antibody causes paroxysmal cold haemoglobinuria with a biphasic IgG pattern, usually after viral illness in children.

Why is "IgE antibody against red cell membrane proteins" not the answer?

IgE antibodies mediate type I reactions and do not cause haemolysis with C3 coating.

What concept does this question test?

It tests Hypersensitivity: type II haemolysis in Pathology. On returning to warmer areas the IgM detaches, leaving C3 on the cells, so the direct antiglobulin test is positive for C3 alone. Haemolysis is mainly extravascular and agglutination causes acrocyanosis.

How hard is this question?

We tag it as a multi-step question: it asks you to combine more than one idea. 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 Hypersensitivity: type II haemolysis?

Learn each disease as a bundle: cause, key histology, markers, and the clinical hallmark. Then use the flashcards and the other Pathology 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.