Natural killer (NK) cells are a type of white blood cell that plays a critical role in the immune system's defense against infections and cancer. Unlike other immune cells, NK cells do not require prior activation to kill infected or abnormal cells. Instead, they detect and target cells that display abnormal or foreign proteins, such as virus-infected cells or tumor cells. NK cells release toxic substances to induce apoptosis (cell death) in these target cells, helping to eliminate threats to the body's health. NK cells are a vital component of innate immunity and contribute to the body's ability to mount an effective immune response.