Q.34 Cytotoxic T cells are :-
(1) CD3+ CD8+
(2) CD3+ CD4+
(3) CD4+ CD8+
(4) CD8+ CD3−
Correct Answer
The correct answer is (1) CD3+ CD8+.
All mature T cells express CD3, a core component of the T cell receptor (TCR) complex essential for antigen recognition and signaling. Cytotoxic T cells (also called CD8+ T cells or killer T cells) specifically express CD8 as a co-receptor, which binds to MHC class I molecules on target cells. This allows them to detect and destroy infected or abnormal cells via perforin and granzymes.
Why CD3+ CD8+ Defines Cytotoxic T Cells
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CD3+: Present on all T lymphocytes (both CD4+ and CD8+). It stabilizes the TCR and transduces activation signals.
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CD8+: Marks cytotoxic T cells. CD8 enhances TCR-MHC I interaction, enabling cytotoxicity.
For example, during a viral infection like influenza, CD3+ CD8+ T cells scan cells presenting viral peptides on MHC I and trigger apoptosis.
Explanation of All Options
Here’s a clear breakdown of why each option is right or wrong:
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Option (1) CD3+ CD8+: Correct. This is the standard phenotype for cytotoxic (CD8+) T cells. They mature in the thymus, express both markers, and function in cell-mediated immunity.
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Option (2) CD3+ CD4+: Incorrect. CD3+ CD4+ describes helper T cells (Th cells), which activate B cells, macrophages, and CD8+ T cells via cytokines. They bind MHC class II, not MHC I.
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Option (3) CD4+ CD8+: Incorrect. This double-positive (DP) stage occurs transiently during T cell development in the thymus, before cells commit to CD4+ or CD8+ single-positive maturity. Mature cytotoxic T cells are never CD4+ CD8+.
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Option (4) CD8+ CD3−: Incorrect. No mature T cells lack CD3—it’s required for TCR function. CD3− CD8+ cells might appear in rare immature or pathological states, but not as functional cytotoxic T cells.
Quick Comparison Table: T Cell Subsets
| Marker | Cell Type | Main Function | MHC Interaction |
|---|---|---|---|
| CD3+ CD8+ | Cytotoxic T cells | Kill infected/cancer cells | Class I |
| CD3+ CD4+ | Helper T cells | Coordinate immune responses | Class II |
| CD4+ CD8+ | Immature thymocytes | T cell development stage | N/A |
| CD8+ CD3− | Not mature T cells | Non-functional or precursor | N/A |
This MCQ tests core immunology knowledge, often seen in exams like NEET, USMLE, or biology graduate tests. Understanding these markers helps grasp T cell roles in immunity.
