Cerebrospinal fluid (CSF) exosomes from GBM patients contain the unique protein‐LGALS9 ligand, which binds to the TIM3 receptor of DCs in CSF and further interferes DCs recognition, inhibits the presentation of antigen, resulting in the deactivation of cytotoxic T cell‐mediated antitumor immune responses. Here, LGALS9 is linked to glioblastoma.