An ancient molecule that existed long before the circulatory system evolved has been found to support cancer immunotherapy. Researchers at Nagoya University in Japan found that complement C3 protein acts inside tumors to prevent the accumulation of immunosuppressive cells, but only when produced inside the tumor. C3 circulating in the blood did not affect treatment outcomes.

Published in Nature Communications, the findings suggest that artificially recreating this effect can help patients with tumors that do not naturally produce enough of this protein.