Cell-mediated immunity: Which immune cell specifically targets and destroys host body cells in which viruses are actively reproducing?

Introduction / Context:
Viruses replicate inside host cells, hiding from many humoral defenses. The immune system therefore relies on cell-mediated mechanisms to identify and eliminate infected cells before new virions are released.

Given Data / Assumptions:

• The question asks for the cell type that defends against virus-infected body cells.
• We compare functions of T-cell subsets and non-cellular toxins.
• Standard mammalian immunology applies.

Concept / Approach:
Cytotoxic T lymphocytes (CTLs, CD8+) recognize infected cells by binding to viral peptides displayed on MHC class I molecules. After recognition, CTLs trigger apoptosis via perforin/granzyme release or Fas–FasL interactions, preventing further viral production.

Step-by-Step Solution:

Verification / Alternative check:
Clinical evidence: deficiency of CD8+ T cells leads to impaired viral clearance. Experimental assays (e.g., chromium release) demonstrate CTL-mediated cytolysis of peptide–MHC I matched targets.

Why Other Options Are Wrong:

• Exotoxin/Endotoxin: toxins, not host immune cells.
• Suppressor (regulatory) T cell: primarily downmodulates immune responses; does not directly kill infected cells.

Common Pitfalls:
Confusing natural killer (NK) cells with CTLs. NK cells kill cells with low MHC I expression, but the specific recognition of viral peptides on MHC I is the role of cytotoxic T cells.

Final Answer:
Cytotoxic T cell