Which immune cells kill virally infected cells by releasing perforin and granzyme?

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

Answer: (C)

Killing virally infected cells with perforin and granzyme is a function of cytotoxic lymphocytes. Perforin creates pores in the target cell membrane, allowing granzyme enzymes to enter and activate the cell’s apoptosis machinery. This mechanism is used by natural killer cells and CD8+ cytotoxic T cells; NK cells, in particular, can do this without prior sensitization and can recognize cells that have downregulated MHC I, a common viral evasion tactic. Among the options, natural killer cells are the ones that primarily use this perforin/granzyme pathway to destroy infected cells. B cells, helper T cells, and macrophages perform other roles—antibody production, cytokine coordination, and phagocytosis/presentation, respectively—and do not kill infected cells via this pathway.