The sophisticated human immune system has evolved to become an effective protection system against a great number of diseases, including cancer. The immune system recognizes and destroys cancer cells using a monitoring process called immunological surveillance. However, like Dr. Jekyll and his alter ego Mr. Hyde, the immune system sometimes takes on an alternate personality and favors tumor development instead of destroying cancerous cells. This dual behavior makes it difficult to detect prognosis indicators and targets for drug development. In fact, one of the big challenges in oncology is the development of better, more effective immunotherapy.