Natural killer (NK) cells are a type of cytotoxic lymphocyte that plays a critical role in the innate immune system. They are able to recognize and kill infected or abnormal cells, such as tumor cells or virus-infected cells, without prior sensitization like other immune cells. NK cells are part of the body's first line of defense against pathogens and are involved in the early immune response to infection. Research on NK cells focuses on understanding their activation and function, as well as their role in various diseases, including cancer, infectious diseases, and autoimmune disorders. Scientists are investigating ways to enhance the activity of NK cells for therapeutic purposes, such as in cancer immunotherapy. Additionally, the study of NK cells can provide insights into the mechanisms of immune regulation and tolerance.