Immunotherapy, which harnesses the power of the body’s immune system to fight disease, is gaining huge traction in treating cancer. Chief among cancer immunotherapies is a treatment known as CAR-T, or chimeric antigen receptor T cell therapy. CAR-T therapy has had great success in treating blood cancers such as leukemia, especially in children and young adults. But using the therapy to combat solid tumors, including skin cancer, has proven much more difficult.