Natural Killer cells are part of the body’s first line of defense against infections and tumors. They are part of the innate immune system. Unlike T-cells and B-cells of the adaptive immune system, which require activation and time to respond to specific antigens, Natural Killer (NK) cells can rapidly target and destroy cells infected with viruses or have become cancerous without prior sensitization. They are called “natural killers” because of their inherent ability to kill without the need for prior exposure to the target. NK cells recognize stressed cells without antibodies and MHC (major histocompatibility complex), allowing for a quicker immune response. They release cytotoxic granules that can induce apoptosis (cell death) in the target cells, thereby preventing the spread of infection and tumorigenesis. Natural killer cells also regulate immune responses through the secretion of cytokines, which can enhance the activity of other immune cells. Various factors, including stress, exercise, and diet, can influence their function and activity, highlighting the importance of maintaining overall health for optimal immune function.