Select the correct statement about adaptive immune recognition and gene generation mechanisms:
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AAll T cells are CD4+ helper cells
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BAn epitope can have multiple antigens
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CT-cell receptors (TCRs) are generated by random assortment and recombination of gene segments (V(D)J recombination)
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DMHC class II molecules are generated by random assortment of rearranging gene cassettes like TCRs
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EB-cell receptors are fixed germline proteins with no gene rearrangement
Answer
Correct Answer: T-cell receptors (TCRs) are generated by random assortment and recombination of gene segments (V(D)J recombination)
Explanation
Introduction / Context:Adaptive immunity relies on vast receptor diversity generated somatically. T-cell receptors (and B-cell receptors/antibodies) arise through V(D)J recombination, junctional diversity, and, for B cells, somatic hypermutation after activation. By contrast, MHC molecules are encoded by polymorphic germline genes without somatic rearrangement.
Given Data / Assumptions:
- TCR and BCR diversity mechanisms are compared to MHC genetics.
- Basic definitions of antigen, epitope, and lineage markers are assumed.
Concept / Approach:TCR α/β chains are formed by recombining variable (V), diversity (D, beta chain only), and joining (J) segments with random N/P nucleotide additions. This produces a unique receptor per T cell. MHC genes are inherited and highly polymorphic but do not undergo somatic rearrangement. Not all T cells are CD4+; many are CD8+.
Step-by-Step Solution:
Step 1: Identify which statement reflects somatic recombination → TCR generation.Step 2: Exclude incorrect claims: “all T cells CD4+” is false; “epitope has multiple antigens” reverses the relationship (an antigen has multiple epitopes); “MHC II by rearrangement” is false; “BCR fixed germline” is false.Step 3: Choose statement (c).Verification / Alternative check:Immunology texts detail RAG1/2-mediated recombination for TCR/BCR; MHC genes are non-rearranging, though polymorphic at the population level.
Why Other Options Are Wrong:
- CD4+ only: CD8+ cytotoxic T cells exist.
- Epitope vs antigen: An antigen contains many epitopes; not vice versa.
- MHC II rearrangement: Does not occur somatically.
- BCR fixed: BCRs undergo V(D)J recombination and, after activation, somatic hypermutation.
Common Pitfalls:
- Confusing population polymorphism (MHC) with somatic recombination (TCR/BCR).
Final Answer:T-cell receptors (TCRs) are generated by random assortment and recombination of gene segments (V(D)J recombination)