What is the main function of natural killer cells in immune defense?

Natural killer (NK) cells are a crucial component of the innate immune system, and their primary role is to directly kill infected or cancerous cells. They do this by recognizing stressed cells in the body, such as those that are infected by viruses or have undergone malignant transformation. Upon identification of these target cells, NK cells release cytotoxic granules that lead to the lysis (destruction) of the target cells.

In contrast to other immune responses that may involve antibody production or phagocytosis of pathogens, NK cells operate without prior sensitization to specific antigens. This allows them to provide a rapid response to a wide variety of infected or abnormal cells. Their ability to recognize and eliminate compromised cells is essential for maintaining cellular integrity and preventing the spread of infections and tumors.

The other functions mentioned, such as producing antibodies or engulfing pathogens, are performed by different components of the immune system, such as B cells and phagocytes, respectively. Activating other immune cells is also a significant function primarily attributed to helper T cells, rather than NK cells.