An immune molecule that appeared in evolution long before blood circulation may help improve cancer immunotherapy. By keeping them out, locally produced C3 gives the immune system a better opportunity to recognize and fight tumor cells. The results point to C3 as an important regulator of cancer immunotherapy, a form of treatment that helps the immune system identify and attack cancer cells. "What determined the efficacy of the immunotherapy treatment was not the C3 in the blood, but the local C3 produced at the tumor site. Those with higher levels of C3 in the tissue surrounding cancer cells experienced better treatment results and longer survival.