Targeting the immune system therapeutically has been a long-standing approach in oncology treatment. The interaction between the immune system and cancer was already discovered in 1863 by Virchow, who hypothesized that sites of chronic inflammation are likely to be the origin of cancer. More recently, immune evasion was announced a hallmark of cancer. In the early phase of tumor induction, the immune system is still able to eliminate most of the cancer-initiating cells. However, the selection pressure for cells circumventing immune responses results in an equilibrium between immune attack and the growing tumor. Eventually, the cancer cells manage to evade the immune response via several immunosuppressive and escape mechanisms.