Researchers have begun to identify the mechanisms that tumors use to protect themselves from the body’s immune system. Disrupting these mechanisms frees the immune system to attack the cancer, offering the hope of effective therapies for otherwise hard-to-treat cancers. Among the first such treatments is ipilimumab (Yervoy), which was approved for treatment of melanoma in 2011. Additional immunotherapy drugs are currently under investigation for lung cancer treatment. Overall, these drugs produce modest increases in average survival. However, some patients respond dramatically: 20% of melanoma patients treated with Yervoy in a clinical trial are still alive up to 10 years later. In others, immunotherapy can cause the immune system to attack healthy cells also, leading to dangerous or even fatal reactions. Further research aims to uncover the reasons behind these different responses.