The immune system is a powerful weapon against disease, infections and defective cells. Because cancer cells are the body’s own mutated cells, they are not always recognized by the immune system as invaders. Also, cancer cells have multiple ways to evade, shut down or overpower an immune attack. Immunotherapy is a broad category of cancer therapies designed to stimulate the body’s immune system to better recognize and fight cancer. Among the most successful immunotherapies used to treat cancer are checkpoint inhibitors that work by disrupting the cancer cells’ signals, exposing them to the immune system for attack.