The human body has developed various mechanisms, through which it can protect itself against newly-developing cancer cells. For instance, killer-cells recognize and destroy altered cells in our organs every day. Once tumors have developed, they may be inhibited in growth by messenger substances from the immune system. Scientists have now succeeded to reveal a completely unexpected function of such an immunological messenger substance in the suppression of tumors -- the molecule beta-interferon inhibits the tumor in its attempts to connect into the human blood circulatory system. Moreover, it hinders the production of growth factors that support the formation of new blood vessels. The conclusion: the tumor cannot grow.