Previous studies have suggested that cancer‐cell‐derived C3 can disrupt the blood‐CSF barrier by activating C3aR in the choroid plexus epithelium.[40] To determine whether C3aR signaling contributes to BBB disruption following anesthesia/surgery, we investigated whether C3aR signaling is required for peripheral C3 to cross the BBB. The gene discussed is C3; the disease is cancer.