Final answer:
NK cells kill infected or tumor cells by inducing apoptosis through the release of proteins like perforin and granzymes.
Step-by-step explanation:
NK cells, also known as natural killer cells, kill infected or tumor cells by inducing programmed cell death, or apoptosis. They do this through the release of proteins like perforin and granzymes. Perforin forms pores in the target cell's membranes, while granzymes enter the cell through these pores and trigger apoptosis intracellularly.