Immunology: principle of the ASO titre: 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: principle of the ASO titre

Sample item

In the antistreptolysin O titration, serial dilutions of patient serum are incubated with a fixed amount of streptolysin O before indicator red cells are added. The highest dilution that prevents haemolysis is reported. Which type of antigen-antibody reaction is this?

ANeutralisationBPrecipitationCComplement fixationDAgglutination

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

The answer and the reasoning

A. Neutralisation

Antibody in the serum binds and neutralises the haemolytic toxin, so the red cells survive. The titre is the highest serum dilution that still prevents lysis. Raised antistreptolysin O titres support recent group A streptococcal infection, for example in rheumatic fever or post-streptococcal glomerulonephritis.

Option by option

A. Neutralisation (correct)
Correct. Antibody neutralises the toxin's biological activity, preventing lysis.
B. Precipitation
Precipitation involves visible precipitate formation of soluble antigen and antibody in gel or fluid, which is not the endpoint.
C. Complement fixation
Complement fixation uses complement and an indicator system, which is not part of this test.
D. Agglutination
Agglutination requires particulate antigen clumping, and here a soluble toxin is neutralised.

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 Microbiology question

What is the answer to this Microbiology question?

The best answer is A, Neutralisation. Antibody in the serum binds and neutralises the haemolytic toxin, so the red cells survive.

Why is "Precipitation" not the answer?

Precipitation involves visible precipitate formation of soluble antigen and antibody in gel or fluid, which is not the endpoint.

Why is "Complement fixation" not the answer?

Complement fixation uses complement and an indicator system, which is not part of this test.

Why is "Agglutination" not the answer?

Agglutination requires particulate antigen clumping, and here a soluble toxin is neutralised.

What concept does this question test?

It tests Immunology: principle of the ASO titre in Microbiology. The titre is the highest serum dilution that still prevents lysis. Raised antistreptolysin O titres support recent group A streptococcal infection, for example in rheumatic fever or post-streptococcal glomerulonephritis.

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: principle of the ASO titre?

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.