Immunology: antibody structure: 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: antibody structure

Sample item

An IgG molecule is digested with pepsin instead of papain. Which fragments are obtained?

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

The answer and the reasoning

B. One divalent F(ab')2 fragment and small Fc pieces

Pepsin cleaves below the hinge disulphide bonds, so the two antigen-binding arms stay joined as a bivalent F(ab')2 fragment that can still cross-link antigen. Papain cleaves above the hinge, giving two monovalent Fab fragments and one crystallisable Fc fragment. Reducing agents are needed to separate heavy from light chains.

Option by option

  • A. Variable domains only

    Proteolytic digestion does not isolate the variable domains alone.

  • B. One divalent F(ab')2 fragment and small Fc pieces (correct)

    Correct. Pepsin cuts below the hinge, leaving the two Fab arms linked.

  • C. Separate heavy and light chains

    Separation of chains needs reduction of disulphide bonds, not pepsin.

  • D. Two monovalent Fab fragments and one intact Fc

    This is the result of papain digestion, which cuts above the hinge.

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 B, One divalent F(ab')2 fragment and small Fc pieces. Pepsin cleaves below the hinge disulphide bonds, so the two antigen-binding arms stay joined as a bivalent F(ab')2 fragment that can still cross-link antigen.

Why is "Variable domains only" not the answer?

Proteolytic digestion does not isolate the variable domains alone.

Why is "Separate heavy and light chains" not the answer?

Separation of chains needs reduction of disulphide bonds, not pepsin.

Why is "Two monovalent Fab fragments and one intact Fc" not the answer?

This is the result of papain digestion, which cuts above the hinge.

What concept does this question test?

It tests Immunology: antibody structure in Microbiology. Papain cleaves above the hinge, giving two monovalent Fab fragments and one crystallisable Fc fragment. Reducing agents are needed to separate heavy from light chains.

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 Immunology: antibody structure?

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.