Immunology: positive selection in the thymus: 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: positive selection in the thymus

Sample item

Immature CD4 and CD8 double-positive thymocytes must recognise self MHC molecules with at least weak affinity or they die by neglect. Where does this positive selection occur, and which cells mediate it?

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

The answer and the reasoning

C. Thymic cortex; cortical epithelial cells

Positive selection occurs in the thymic cortex, where cortical epithelial cells present self peptide-MHC complexes to double-positive thymocytes. Cells that bind with moderate affinity survive and become single-positive and MHC restricted. Strongly self-reactive cells are removed later by negative selection in the medulla.

Option by option

  • A. Subcapsular zone; macrophages

    The subcapsular zone is where the earliest precursors enter, not where selection by MHC occurs.

  • B. Thymic medulla; medullary epithelial cells expressing AIRE

    The medulla with AIRE-expressing epithelial cells is the site of negative selection for self-reactive cells.

  • C. Thymic cortex; cortical epithelial cells (correct)

    Correct. Cortical epithelial cells support survival of thymocytes with useful self-MHC recognition.

  • D. Peripheral lymph node; dendritic cells

    Peripheral nodes are secondary lymphoid organs where mature T cells meet antigen, and thymic selection does not occur there.

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 C, Thymic cortex; cortical epithelial cells. Positive selection occurs in the thymic cortex, where cortical epithelial cells present self peptide-MHC complexes to double-positive thymocytes.

Why is "Subcapsular zone; macrophages" not the answer?

The subcapsular zone is where the earliest precursors enter, not where selection by MHC occurs.

Why is "Thymic medulla; medullary epithelial cells expressing AIRE" not the answer?

The medulla with AIRE-expressing epithelial cells is the site of negative selection for self-reactive cells.

Why is "Peripheral lymph node; dendritic cells" not the answer?

Peripheral nodes are secondary lymphoid organs where mature T cells meet antigen, and thymic selection does not occur there.

What concept does this question test?

It tests Immunology: positive selection in the thymus in Microbiology. Cells that bind with moderate affinity survive and become single-positive and MHC restricted. Strongly self-reactive cells are removed later by negative selection in the medulla.

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: positive selection in the thymus?

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.