Cancer is a major cause of death worldwide. Immunotherapy is one of the most promising new ways to treat advanced stages of cancer. It works by “taking the breaks” off the immune system to let immune cells kill cancer cells better. Immunotherapy has revolutionized the treatment of cancers like melanoma, lung cancer and bladder cancer. Still, many patients do not respond to treatment. It is hard to know early who will respond and who won’t. We are developing and testing a method to predict response to immunotherapy early. We are doing this through a simple blood test that measures signal from immune cells deep inside a patient’s tumor. We are testing our method in melanoma patients. If successful, our method will revolutionize the ability to predict cancer response to immunotherapy. This will give doctors vital information early and improve patient survival.