Immunology: single radial immunodiffusion: 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: single radial immunodiffusion

Sample item

A laboratory measures serum IgG by placing patient serum in wells cut into agar containing anti-human IgG. After 24 hours, circular precipitin rings form, and the squares of their diameters are compared with a standard curve. Which technique is this?

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

The answer and the reasoning

D. Single radial immunodiffusion (Mancini)

In single radial immunodiffusion, antigen diffuses outward from a well into agar containing antibody, and the area of the precipitation ring (diameter squared) is proportional to antigen concentration. It was widely used for quantitating immunoglobulins and complement components. Ouchterlony double diffusion compares antigens qualitatively.

Option by option

  • A. Double diffusion (Ouchterlony)

    Ouchterlony double diffusion has both antigen and antibody diffusing and gives lines of identity or non-identity rather than quantitation.

  • B. Immunofluorescence microscopy

    Immunofluorescence relies on labelled antibody and microscopy, not on precipitation rings in agar.

  • C. Counter-current gel assay

    Counter-current methods use an electric field to drive antigen and antibody toward each other for rapid qualitative detection.

  • D. Single radial immunodiffusion (Mancini) (correct)

    Correct. Ring area against a standard curve gives quantitative measurement in Mancini's method.

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 D, Single radial immunodiffusion (Mancini). In single radial immunodiffusion, antigen diffuses outward from a well into agar containing antibody, and the area of the precipitation ring (diameter squared) is proportional to antigen concentration.

Why is "Double diffusion (Ouchterlony)" not the answer?

Ouchterlony double diffusion has both antigen and antibody diffusing and gives lines of identity or non-identity rather than quantitation.

Why is "Immunofluorescence microscopy" not the answer?

Immunofluorescence relies on labelled antibody and microscopy, not on precipitation rings in agar.

Why is "Counter-current gel assay" not the answer?

Counter-current methods use an electric field to drive antigen and antibody toward each other for rapid qualitative detection.

What concept does this question test?

It tests Immunology: single radial immunodiffusion in Microbiology. It was widely used for quantitating immunoglobulins and complement components. Ouchterlony double diffusion compares antigens qualitatively.

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: single radial immunodiffusion?

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.