Immunity: class switching: 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: class switching

Sample item

A 2-year-old boy has recurrent sinopulmonary infections and Pneumocystis pneumonia. Serum IgM is elevated, IgG and IgA are very low, and B-cell numbers are normal. Which defect explains this?

AAbsent thymic development with failure of T-cell productionBDeficiency of adenosine deaminase with toxic metabolite accumulationCDefective CD40 ligand on activated helper T cells, preventing immunoglobulin class switchingDMutation of Bruton tyrosine kinase, blocking pre-B cell maturation

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

The answer and the reasoning

C. Defective CD40 ligand on activated helper T cells, preventing immunoglobulin class switching

CD40 ligand on activated CD4 T cells engages CD40 on B cells, which is required for isotype switching and germinal centre formation. In X-linked hyper-IgM syndrome, B cells make IgM only, and impaired T-cell help to macrophages predisposes to Pneumocystis. B-cell numbers are normal.

Option by option

A. Absent thymic development with failure of T-cell production
Thymic aplasia (DiGeorge syndrome) reduces T cells, but immunoglobulin classes do not show this raised IgM pattern.
B. Deficiency of adenosine deaminase with toxic metabolite accumulation
ADA deficiency causes severe combined immunodeficiency with low T, B and NK cells.
C. Defective CD40 ligand on activated helper T cells, preventing immunoglobulin class switching (correct)
Correct. Without CD40-CD40L signalling, B cells cannot switch from IgM to IgG or IgA.
D. Mutation of Bruton tyrosine kinase, blocking pre-B cell maturation
Bruton agammaglobulinaemia produces very few B cells and low levels of all immunoglobulins, including IgM.

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.

Questions about this Physiology question

What is the answer to this Physiology question?

The best answer is C, Defective CD40 ligand on activated helper T cells, preventing immunoglobulin class switching. CD40 ligand on activated CD4 T cells engages CD40 on B cells, which is required for isotype switching and germinal centre formation.

Why is "Absent thymic development with failure of T-cell production" not the answer?

Thymic aplasia (DiGeorge syndrome) reduces T cells, but immunoglobulin classes do not show this raised IgM pattern.

Why is "Deficiency of adenosine deaminase with toxic metabolite accumulation" not the answer?

ADA deficiency causes severe combined immunodeficiency with low T, B and NK cells.

Why is "Mutation of Bruton tyrosine kinase, blocking pre-B cell maturation" not the answer?

Bruton agammaglobulinaemia produces very few B cells and low levels of all immunoglobulins, including IgM.

What concept does this question test?

It tests Immunity: class switching in Physiology. In X-linked hyper-IgM syndrome, B cells make IgM only, and impaired T-cell help to macrophages predisposes to Pneumocystis. B-cell numbers are normal.

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: class switching?

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.