Natural killer cells, or NK cells, are a type of white blood cell that play a crucial role in the immune system. They are part of the body's first line of defense against infections and tumors. Unlike other immune cells, NK cells can recognize and destroy infected or cancerous cells without needing prior exposure to them. These cells identify harmful cells by detecting changes in their surface markers. When NK cells find a target, they release substances that can induce cell death, helping to control infections and prevent the spread of tumors. Their rapid response makes them essential for maintaining overall health.