Answer:
killing of virus-infected cells
Explanation:
Natural killer (NK) cells is the major component of immune system and a type of white blood cells that play an important role in immunity. Protease enzyme is present in the NK cells.
Natural killer cells can identify the virus infected cells and activates the cascade mechanism to kill the virus cells. Natural killer cells release cytokines that can kill the virus infected cells.
Thus, the correct answer is option 4.
