Final answer:
The mechanism of the "lethal hit" of cytotoxic T cells and NK cells involves the protein perforin. Perforin forms pores in the target cell, allowing granzymes to enter and trigger apoptosis.
Step-by-step explanation:
The mechanism of the "lethal hit" of cytotoxic T cells and NK cells involves a protein called perforin. Perforin is a destructive protein that creates pores in the target cell. When a cytotoxic T cell or NK cell detects an infected or tumor cell, it secretes granules comprised of perforin, which form pores in the cell membrane. This allows granzymes, protein-digesting enzymes, to enter the cell and trigger apoptosis, or programmed cell death.