Immunology: hypersensitivity type III: 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: hypersensitivity type III

Sample item

A 30-year-old man bitten by a snake receives equine antivenom. Nine days later he develops fever, a generalised urticarial rash, joint pains and proteinuria, and serum C3 is low. What is the underlying mechanism?

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

The answer and the reasoning

A. Deposition of circulating antigen-antibody complexes with complement activation

Serum sickness is a type III hypersensitivity in which antibodies to foreign serum proteins form soluble immune complexes that lodge in vessels, joints and glomeruli. Complement activation consumes C3 and C4, and symptoms begin 7 to 14 days after exposure, when antibody appears while antigen is still present.

Option by option

  • A. Deposition of circulating antigen-antibody complexes with complement activation (correct)

    Correct. Soluble immune complexes with complement consumption explain the delayed onset and low C3.

  • B. IgE-mediated degranulation of tissue mast cells

    IgE-mediated mast cell degranulation is type I and produces anaphylaxis within minutes, not after 9 days.

  • C. Sensitised CD4 T cells releasing cytokines at the site

    Sensitised T cells cause type IV delayed hypersensitivity such as the tuberculin reaction, and do not lower C3.

  • D. Antibody binding to antigens on his own cell surfaces

    Antibody against cell-surface antigens is type II, as in haemolytic anaemia or Goodpasture disease.

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 A, Deposition of circulating antigen-antibody complexes with complement activation. Serum sickness is a type III hypersensitivity in which antibodies to foreign serum proteins form soluble immune complexes that lodge in vessels, joints and glomeruli.

Why is "IgE-mediated degranulation of tissue mast cells" not the answer?

IgE-mediated mast cell degranulation is type I and produces anaphylaxis within minutes, not after 9 days.

Why is "Sensitised CD4 T cells releasing cytokines at the site" not the answer?

Sensitised T cells cause type IV delayed hypersensitivity such as the tuberculin reaction, and do not lower C3.

Why is "Antibody binding to antigens on his own cell surfaces" not the answer?

Antibody against cell-surface antigens is type II, as in haemolytic anaemia or Goodpasture disease.

What concept does this question test?

It tests Immunology: hypersensitivity type III in Microbiology. Complement activation consumes C3 and C4, and symptoms begin 7 to 14 days after exposure, when antibody appears while antigen is still present.

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: hypersensitivity type III?

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.