Immunity: immunoglobulin 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: immunoglobulin class switching

Sample item

A 3-year-old boy has recurrent pyogenic sinopulmonary infections and Pneumocystis pneumonia. Serum IgM is high, while IgG, IgA and IgE are very low. Which molecule is most likely defective?

AAdenosine deaminase in lymphocytesBBruton tyrosine kinase in pre-B cellsCWiskott–Aldrich syndrome protein in platelets and T cellsDCD40 ligand on activated CD4 T cells

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

The answer and the reasoning

D. CD40 ligand on activated CD4 T cells

Class switching from IgM to IgG, IgA and IgE requires CD40 ligand on helper T cells engaging CD40 on B cells, together with cytokines. A defect produces X-linked hyper-IgM syndrome, with high or normal IgM and low other isotypes. Opportunistic infection arises because the CD40 pathway also activates macrophages.

Option by option

A. Adenosine deaminase in lymphocytes
Adenosine deaminase deficiency causes severe combined immunodeficiency with low T and B cells.
B. Bruton tyrosine kinase in pre-B cells
A Bruton kinase defect gives low levels of all immunoglobulins, including IgM.
C. Wiskott–Aldrich syndrome protein in platelets and T cells
Wiskott–Aldrich syndrome is marked by eczema and thrombocytopenia, with low IgM.
D. CD40 ligand on activated CD4 T cells (correct)
Correct. Without CD40L, B cells cannot switch isotype or form germinal centres.

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 D, CD40 ligand on activated CD4 T cells. Class switching from IgM to IgG, IgA and IgE requires CD40 ligand on helper T cells engaging CD40 on B cells, together with cytokines.

Why is "Adenosine deaminase in lymphocytes" not the answer?

Adenosine deaminase deficiency causes severe combined immunodeficiency with low T and B cells.

Why is "Bruton tyrosine kinase in pre-B cells" not the answer?

A Bruton kinase defect gives low levels of all immunoglobulins, including IgM.

Why is "Wiskott–Aldrich syndrome protein in platelets and T cells" not the answer?

Wiskott–Aldrich syndrome is marked by eczema and thrombocytopenia, with low IgM.

What concept does this question test?

It tests Immunity: immunoglobulin class switching in Physiology. A defect produces X-linked hyper-IgM syndrome, with high or normal IgM and low other isotypes. Opportunistic infection arises because the CD40 pathway also activates macrophages.

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 Immunity: immunoglobulin 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.