In recent years, immunotherapy has revolutionized the field of cancer treatment. However, inflammatory reactions in healthy tissues frequently trigger side effects that can be serious and lead to the permanent discontinuation of treatment. This toxicity is still poorly understood and is a major obstacle to the use of immunotherapy. Scientists from the University of Geneva (UNIGE), Switzerland, and Harvard Medical School have succeeded in establishing the differences between deleterious immune reactions and those targeting tumor cells. While the immune mechanisms are similar, the cell populations involved are different. This work, published in the journal Science Immunology, could lead to better targeted, more effective, and less dangerous treatments for cancer patients.