Immunology: IgG2 subclass deficiency: practice question

A Microbiology 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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Microbiology · Immunology: IgG2 subclass deficiency

Sample item

A 6-year-old has recurrent otitis media and pneumococcal pneumonia. Serum IgG, IgM and IgA levels are in the normal range, but there is no antibody rise after pneumococcal polysaccharide vaccination. Which defect best explains this?

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

The answer and the reasoning

C. IgG2 subclass deficiency

IgG2 is the main IgG subclass directed against bacterial capsular polysaccharides, and it contributes only a modest fraction of total IgG, so total IgG can be normal in its deficiency. Affected children get infections with encapsulated bacteria and respond poorly to plain polysaccharide vaccines. Conjugate vaccines may help.

Option by option

  • A. IgG1 subclass deficiency

    IgG1 is the most abundant subclass, and its deficiency would lower total IgG noticeably.

  • B. IgE deficiency

    IgE is involved in allergy and helminth defence rather than defence against encapsulated bacteria.

  • C. IgG2 subclass deficiency (correct)

    Correct. IgG2 mediates the response to polysaccharide antigens and its loss may not alter total IgG much.

  • D. IgG3 subclass deficiency

    IgG3 is mainly directed against protein antigens and is a strong complement activator, so it does not explain the failed polysaccharide response.

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 Microbiology question

What is the answer to this Microbiology question?

The best answer is C, IgG2 subclass deficiency. IgG2 is the main IgG subclass directed against bacterial capsular polysaccharides, and it contributes only a modest fraction of total IgG, so total IgG can be normal in its deficiency.

Why is "IgG1 subclass deficiency" not the answer?

IgG1 is the most abundant subclass, and its deficiency would lower total IgG noticeably.

Why is "IgE deficiency" not the answer?

IgE is involved in allergy and helminth defence rather than defence against encapsulated bacteria.

Why is "IgG3 subclass deficiency" not the answer?

IgG3 is mainly directed against protein antigens and is a strong complement activator, so it does not explain the failed polysaccharide response.

What concept does this question test?

It tests Immunology: IgG2 subclass deficiency in Microbiology. Affected children get infections with encapsulated bacteria and respond poorly to plain polysaccharide vaccines. Conjugate vaccines may help.

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 Immunology: IgG2 subclass deficiency?

Make a one-line identity card for each organism: shape and stain, defining test, toxin, disease and drug. Then use the flashcards and the other Microbiology 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.