What is the main effector immune cell of a viral infection?

Question

BlackTom AI · Accepted Answer

Cytotoxic (CD8+) T lymphocytes are the principal effector cells that recognize and kill virus-infected host cells.

1) Relevant knowledge points and concepts
- Viruses are intracellular pathogens that replicate inside host cells; eliminating infected cells is essential to stop replication.
- Adaptive cellular immunity: CD8+ (cytotoxic) T cells recognize viral peptides presented on MHC class I molecules on infected cells.
- Effector mechanisms of CD8+ T cells include release of perforin and granzymes (inducing apoptosis), Fas–FasL interactions, and production of antiviral cytokines (e.g., IFN-γ).

2) Step-by-step reasoning
- Viral infection results in synthesis of viral proteins inside host cells.
- Peptides from these proteins are loaded on MHC I and displayed on the infected cell surface.
- Naive CD8+ T cells, once primed by antigen-presenting cells and helper signals, differentiate into cytotoxic effector T cells.
- Effector CD8+ T cells patrol tissues, recognize infected cells via T-cell receptor–MHC I interaction, and directly kill those cells, thereby removing sites of viral replication.

3) Calculations/intermediate steps
- No numerical calculations are required. Mechanistic sequence: infection → antigen presentation on MHC I → CD8+ activation → target cell killing.

4) Why the correct answer is correct
- Because viruses hide within host cells, neutralizing antibodies alone cannot clear all intracellular virus. Cytotoxic T cells are specialized to find and destroy infected host cells, making them the principal effector cells for viral clearance.

5) Why other options are incorrect
- a. Neutrophils: key for early innate defense against extracellular bacteria and fungi (phagocytosis), not specialized for clearing intracellular viruses.
- b. B cells: produce antibodies that neutralize extracellular virus and prevent entry, but they do not eliminate already infected cells as the main effector.
- c. Antigen-presenting cells: essential for activating T cells but are not the primary effectors that kill infected cells.
- e. Eosinophils: mainly involved in parasitic helminth defense and allergic responses, not antiviral cytotoxicity.

6) Conclusion
The correct answer is d. Cytotoxic T cells, which matches the provided answer.

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