NK cells of the innate immune system have two primary functions: to kill virally infected cells and to kill cancer cells. We focus on their role in killing cancer cells. NK cells play an important role in preventing cancer. That is, every day our bodies make precancerous cells. Our ever vigilant immune system seeks these abnormal cells. When an NK cell contacts a precancerous cell, it kills it. This process, called Immune Surveillance, keeps us cancer free. An NK cell must come in direct contact with the cancer cell to kill it. The NK cells secrete granzyme into the cancer cell, which causes holes in the cell membrane and kills the cell.