(A) P and R

(B) Q and R

(C) Q and S

(D) Q and P

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In B cell development stages, understanding key markers like Terminal deoxynucleotidyl transferase (TdT), class switching, surrogate light chain, and Recombination Activating Gene 1 (RAG1) is crucial for immunology exams and biotechnology studies. This article breaks down MCQ Q.40, reveals the correct answer, and explains all options with molecular biology details to help with revision.

This question tests your grasp of B cell maturation stages in the bone marrow and periphery, focusing on V(D)J recombination, immunoglobulin gene rearrangement, and differentiation markers.

Correct Answer: (B) Q and R

The correctly matched pairs are Q (Activated B cells — Class switching) and R (Pre B cells — Surrogate light chain).

Why Q is Correct: Activated B Cells and Class Switching

Activated B cells undergo class switching (also called isotype switching) upon antigen stimulation and T cell help. This process involves cytokine signals (e.g., IL-4 for IgE) that activate Activation-Induced Cytidine Deaminase (AID), leading to DNA recombination between switch regions. It changes the constant region of the heavy chain (e.g., IgM to IgG) without altering antigen specificity. This hallmark occurs in germinal centers post-activation, not in early bone marrow stages.

Why R is Correct: Pre-B Cells and Surrogate Light Chain

Pre-B cells (pre-BCR stage) express a surrogate light chain (VpreB and λ5 proteins) paired with the μ heavy chain to form the pre-B cell receptor (pre-BCR). This surrogate mimics a full BCR, signaling proliferation and light chain rearrangement. It’s essential in early B cell development in bone marrow, absent in later stages.

Explanation of Incorrect Options: Why P and S Don’t Match

Option (A) P and R: Partially Correct, But P is Wrong

• R is correct (as above).

• P (Immature B cells — TdT) is incorrect. TdT adds N-nucleotides during V(D)J recombination in pro-B and early pre-B cells for junctional diversity. Expression peaks early and declines by the immature B cell stage, where light chain rearrangement completes and full BCR (IgM) appears on the surface. TdT is off in immature B cells to prevent further heavy chain diversity.

Option (C) Q and S: Q Correct, S Wrong

• Q is correct.

• S (Mature B cells — RAG1) is incorrect. RAG1 (and RAG2) drive V(D)J recombination in pro-B and pre-B cells. In mature B cells (surface IgM+ IgD+), RAG expression is downregulated to avoid further rearrangements, ensuring allelic exclusion and stable BCR.

Option (D) Q and P: Both Wrong for Matching

• Q is correct, but P mismatches as explained.

Key Takeaways for B Cell Development Exams

• Pro-B/Pre-B: RAG1/2, TdT, surrogate light chain (VpreB/λ5).

• Immature: Full BCR (IgM), no TdT/RAG.

• Mature: IgM/IgD, ready for activation.

• Activated: Somatic hypermutation, class switching.

Master these for NEET PG, CSIR NET, or biotech interviews. For more on immunoglobulin gene rearrangement or B cell markers, check related articles.