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GATE 2020 BT Q.4: solution and answer

GATE 2020 · BTQ.4 · 1 mark · MCQ

Which class of antibody is first made by developing B cells inside bone marrow?

  1. (A)

    IgG

  2. (B)

    IgE

  3. (C)

    IgA

  4. (D)

    IgM

See the question in the paper
GATE 2020 BT question 4Tap to open full size

Answer and solution

ANSWER

The correct option is (D) IgM.

Official answer: DSource: Official answer key, GATE 2020 BT (IIT Delhi)

  1. In the bone marrow, a developing B cell first rearranges its heavy-chain genes (D-J, then V-DJ joining). The constant region () lies closest to the V-D-J segments, so the first heavy chain made is .

  2. Pre-B cells make heavy chains and pair them with surrogate light chains. After the light-chain genes rearrange, the immature B cell displays membrane IgM. This is the first complete antibody made, and it is made before any antigen is met.

  3. A mature naive B cell leaving the marrow also makes IgD, by alternative splicing of one long heavy-chain transcript. IgM is still the first class made.

  4. (A) IgG is wrong. It is made only after antigen activation and class switching, which take place in secondary lymphoid organs and need helper T-cell signals.

  5. (B) IgE is wrong. It also arises by class switching after antigen exposure.

  6. (C) IgA is wrong. It is made after class switching, mainly in mucosal lymphoid tissue.

  7. (D) IgM is right. It is the first isotype made by developing B cells in the bone marrow, and no antigen is needed for it.

KEY IDEA

The gene is the closest constant region to the V-D-J segments, so a developing B cell makes IgM first. Class switching to IgG, IgE or IgA comes later, after antigen exposure outside the marrow.

WATCH OUT FOR

Picking IgG because it is the most abundant antibody in serum. Abundance has nothing to do with which class is made first. IgG, IgE and IgA all need antigen-driven class switching.

From the GATE 2020 BT paper. Every question of this paper · Download the full paper · All 6 Immune system questions, with notes

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