Short-term cultivated, individualised immune cells (CAR T cells) are currently being developed as a therapeutic option for patients with blood cancer. A research team from the Paul-Ehrlich-Institut has shown with mouse and cell models that these cells carry a higher risk for cytokine release syndrome than conventional CAR T cells. The cytokine release is caused by residual components of vector particles on the CAR T cells and is independent of tumour cells. Careful consideration of the safety of this innovative treatment is required to minimise risks to patients. EMBO Molecular Medicine reports on the results in its issue dated 21 March 2024.