 Around 40 million. That's the number of people affected by cancer worldwide. Cancer arises when DNA mutations cause a cell to divide uncontrollably and behave abnormally. So, even though a cancer cell starts out as a normal cell, because it accumulates mutations over time, it transforms into something that looks foreign. Our bodies already have a system in place, the immune system, whose job it is to look for and kill foreign invaders. The usual invaders are bacteria, viruses and parasites, but researchers started to think what if we could engage our immune system to not just search out pathogens, but to also look for and kill the foreign-looking cancer cells. This idea sparked a revolution in cancer research, the growing field of cancer immunotherapy. Using the immune system to kill cancer cells has become a very exciting and promising treatment strategy for cancer. So, how exactly do cancer immunotherapies work? To understand this, we need to understand how the cells of our immune system look for and then kill foreign invaders. The main killing cells of our immune system are called T-cells. These immune cells are constantly moving around our bodies, and they make contacts with other cells they encounter to see whether these cells look normal or if they look foreign. Immune cells do this by using molecules on their surface that recognize proteins on the other cells. If the T-cell makes contact and recognizes something foreign, like a virus infected cell, it gets a green light for going on the attack, and the immune cell will kill the infected cell. To make sure that our immune cells don't get out of control and damage healthy cells, some molecules on the T-cells act as breaks, providing a counteracting force that slows the immune attack down. These molecules which tell the immune cells whether to stop or go are called immune checkpoints. So, what does this have to do with cancer treatment? Well, cancer cells look a bit like foreign cells because they have lots of DNA changes, some of which result in altered proteins. However, cancer cells are sneaky, and in order to avoid triggering a green light attack response from the T-cells, they make proteins that elicit the red light response. By doing this, the cancer cells trick the immune cells into stepping on the breaks instead of on the accelerator pedal. Back in the mid-1990s, several scientists including doctors James, Allison, and Tzuku Honjo realized that blocking the ability of the immune cells to see the red light checkpoint on the cancer cells would cause the immune cells to attack the cancer cells. Researchers then designed antibodies that recognize and specifically block the proteins that send out the red light signal. With the red light signal suppressed, these checkpoint inhibitor therapies allow immune cells to kill cancer cells more effectively. This strategy has worked remarkably well. Checkpoint inhibitor therapy was approved by the FDA as a treatment for cancer in 2011, and in 2018, led to the Nobel Prize being awarded to Allison and Honjo for pioneering this approach. Another promising cancer immunotherapy strategy has been to enhance the green light on T-cells, a method called CAR-T therapy. A patient's T-cells are collected and a new gene is introduced. This new gene customizes and reprograms T-cells so that when they are put back into the patient, they specifically recognize a particular molecule on the cancer cells. These CAR-T cells seek out and destroy cancer cells that display that molecule. This patient-specific customized cancer immunotherapy has also been recently approved by the Food and Drug Administration. Despite their success, immunotherapies are not without side effects. Some patients can develop autoimmune diseases and inflammation. Cancer cells also have ways of evading immunotherapies. In the battle of the cells, immune cells can win initially, but later on, the cancer cells can become resistant and the disease may return. Because of this, scientists and physicians are hard at work trying to find ways to make immunotherapies as safe and effective as possible. Perhaps we have just scratched the surface of what our immune system can do. In the battle of the cells, perhaps cancer cells might lose out in the end.