Immunology: affinity maturation: 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: affinity maturation

Sample item

Over successive tetanus toxoid boosters, a child's antibodies bind the toxin with progressively higher affinity, and sequencing of antibody genes from B cells shows many point mutations in the variable regions. Which enzyme is responsible and where does the process occur?

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

The answer and the reasoning

C. Activation-induced cytidine deaminase in germinal centres

Antigen-activated B cells in germinal centres undergo somatic hypermutation of variable region genes through activation-induced cytidine deaminase. Follicular dendritic cells and T follicular helper cells then select clones with higher affinity, which is affinity maturation. The same enzyme drives class switch recombination.

Option by option

  • A. Terminal deoxynucleotidyl transferase in thymic medulla

    Terminal deoxynucleotidyl transferase adds nucleotides at junctions during early receptor assembly, not after antigen contact.

  • B. RAG1 and RAG2 recombinases in bone marrow pro-B cells

    RAG recombinases perform antigen-independent V(D)J recombination in the bone marrow and do not cause point mutations after antigen exposure.

  • C. Activation-induced cytidine deaminase in germinal centres (correct)

    Correct. This deaminase introduces point mutations in V genes of germinal centre B cells, and selection gives higher affinity.

  • D. Perforin in the lymph node paracortex

    Perforin is a cytotoxic granule protein that forms pores in target cells and does not mutate immunoglobulin genes.

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, Activation-induced cytidine deaminase in germinal centres. Antigen-activated B cells in germinal centres undergo somatic hypermutation of variable region genes through activation-induced cytidine deaminase.

Why is "Terminal deoxynucleotidyl transferase in thymic medulla" not the answer?

Terminal deoxynucleotidyl transferase adds nucleotides at junctions during early receptor assembly, not after antigen contact.

Why is "RAG1 and RAG2 recombinases in bone marrow pro-B cells" not the answer?

RAG recombinases perform antigen-independent V(D)J recombination in the bone marrow and do not cause point mutations after antigen exposure.

Why is "Perforin in the lymph node paracortex" not the answer?

Perforin is a cytotoxic granule protein that forms pores in target cells and does not mutate immunoglobulin genes.

What concept does this question test?

It tests Immunology: affinity maturation in Microbiology. Follicular dendritic cells and T follicular helper cells then select clones with higher affinity, which is affinity maturation. The same enzyme drives class switch recombination.

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: affinity maturation?

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.