NK cells, or natural killer 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. NK cells can recognize and destroy cells that are infected with viruses or have become cancerous without needing prior sensitization. These cells are unique because they can act quickly and do not require the presence of antibodies to target harmful cells. By releasing cytotoxic granules, NK cells can induce apoptosis, or programmed cell death, in their targets, helping to maintain the body's health and prevent disease progression.