Our immune system has evolved many different cells with different functions to prevent infections. One of these functions is cellular cytotoxicity; this means that cells are able to kill other cells. Natural killer cells (NK cells) are one type of these cells that can achieve cytotoxicity. CARSTEN WATZL experiments with NK cells in order to find out how they protect themselves from their own cytotoxic machinery and how they undertake ‘serial killing’. The research group’s results suggest that the molecule CD107a, which is inside the vesicles that contain the cytotoxic molecules, is involved in protecting the NK cell. The researchers also found evidence that the killing process of NK cells is regulated and that the cells know when they have killed another cell. Their discoveries have important applications for the development of treatments for diseases like cancers and autoimmune diseases.