Immunology: sandwich ELISA for antigen: 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: sandwich ELISA for antigen

Sample item

A blood bank screens donor serum for hepatitis B surface antigen by a sandwich ELISA. What is coated on the wells at the beginning of the assay?

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

The answer and the reasoning

B. Antibody to the surface antigen

In a sandwich ELISA for antigen, wells are coated with capture antibody, the sample antigen binds it, and a second enzyme-labelled antibody binds a different epitope, producing colour with substrate. An indirect ELISA, with antigen on the plate and anti-human immunoglobulin conjugate, is used to detect antibodies.

Option by option

  • A. Enzyme-labelled anti-human immunoglobulin

    An enzyme-labelled anti-human immunoglobulin is the detection reagent in antibody assays, and it is added later rather than coated.

  • B. Antibody to the surface antigen (correct)

    Correct. Capture antibody on the solid phase binds the antigen from the sample.

  • C. Purified surface antigen

    Antigen-coated wells are used in antibody-detecting formats such as indirect ELISA.

  • D. Patient serum antibodies

    The patient's antibodies are the sample being tested, and they are not pre-coated on the plate.

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.

More Microbiology questions

Questions about this Microbiology question

What is the answer to this Microbiology question?

The best answer is B, Antibody to the surface antigen. In a sandwich ELISA for antigen, wells are coated with capture antibody, the sample antigen binds it, and a second enzyme-labelled antibody binds a different epitope, producing colour with substrate.

Why is "Enzyme-labelled anti-human immunoglobulin" not the answer?

An enzyme-labelled anti-human immunoglobulin is the detection reagent in antibody assays, and it is added later rather than coated.

Why is "Purified surface antigen" not the answer?

Antigen-coated wells are used in antibody-detecting formats such as indirect ELISA.

Why is "Patient serum antibodies" not the answer?

The patient's antibodies are the sample being tested, and they are not pre-coated on the plate.

What concept does this question test?

It tests Immunology: sandwich ELISA for antigen in Microbiology. An indirect ELISA, with antigen on the plate and anti-human immunoglobulin conjugate, is used to detect antibodies.

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: sandwich ELISA for antigen?

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.