Immunology: Ouchterlony double diffusion: 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.

Try it

Microbiology · Immunology: Ouchterlony double diffusion

Sample item

In an Ouchterlony double diffusion plate, antiserum is placed in the central well and two antigen preparations in adjacent wells. The two precipitin lines meet and fuse smoothly into one continuous arc. What does this pattern indicate?

AIdentity: both antigens share the same epitopesBPartial identity: a spur extends beyond the fused lineCNon-identity: the two lines cross without interactingDProzone: antigen excess prevents any precipitate

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

The answer and the reasoning

A. Identity: both antigens share the same epitopes

In double immunodiffusion, a smooth continuous arc between adjacent wells shows a reaction of identity. Crossed lines show non-identity and a spur on the fused line shows partial identity, where one antigen has extra determinants.

Option by option

A. Identity: both antigens share the same epitopes (correct)
Correct. A smooth fused arc is a reaction of identity.
B. Partial identity: a spur extends beyond the fused line
Partial identity gives fusion plus a spur projecting past the line.
C. Non-identity: the two lines cross without interacting
Non-identity gives two lines that cross each other without fusing.
D. Prozone: antigen excess prevents any precipitate
A prozone effect produces absent or weak precipitation, not a clean arc.

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, Identity: both antigens share the same epitopes. In double immunodiffusion, a smooth continuous arc between adjacent wells shows a reaction of identity.

Why is "Partial identity: a spur extends beyond the fused line" not the answer?

Partial identity gives fusion plus a spur projecting past the line.

Why is "Non-identity: the two lines cross without interacting" not the answer?

Non-identity gives two lines that cross each other without fusing.

Why is "Prozone: antigen excess prevents any precipitate" not the answer?

A prozone effect produces absent or weak precipitation, not a clean arc.

What concept does this question test?

It tests Immunology: Ouchterlony double diffusion in Microbiology. Crossed lines show non-identity and a spur on the fused line shows partial identity, where one antigen has extra determinants.

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: Ouchterlony double diffusion?

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.