Activated T cells that carry a certain marker protein on their surface are controlled by natural killer (NK) cells, another cell type of the immune system. In this way, the body presumably curbs destructive immune reactions. Researchers have now discovered that NK cells can impair the effect of cancer therapies with immune checkpoint inhibitors (ICI) in this way. They could also be responsible for the rapid decline of therapeutic CAR-T cells. Interventions in this mechanism could potentially improve the efficacy of these cellular cancer immunotherapies.