Virus-infected cells, foreign cells, and cancer cells are all combatted by _________.

• A. B cells
• B. Natural killer cells
• C. Macrophages
• D. T cells

Answer: (D)

Cytotoxic T cells are the main players here. They recognize abnormal peptides presented on MHC class I molecules by nearly all nucleated cells. When a cell is virus-infected or has become cancerous, it displays these abnormal peptides, and cytotoxic T cells bind them and induce the cell’s death to stop the infection or tumor growth. They also target cells from another individual (foreign cells) because the foreign MHC antigens are seen as non-self, triggering a targeted attack. So, this combination—virus-infected, foreign, and cancer cells—is best explained by cytotoxic T cells through antigen-specific recognition and apoptosis of the target cell.