What characteristic defines immunotherapy's mechanism of action?

The characteristic that defines immunotherapy's mechanism of action is its ability to restore or modulate the immune response. Immunotherapy works by enhancing the body's natural immune system to recognize and fight cancer cells effectively. Unlike traditional treatments that may directly target or destroy cancer cells, immunotherapy engages and activates immune cells, such as T-cells and natural killer cells, to identify and eliminate cancerous tissues. This modulation can involve various approaches, such as checkpoint inhibitors, which help to release the brakes on the immune system, allowing it to attack the tumor more vigorously, or therapeutic vaccines that prime the immune system against specific cancer antigens.

In contrast, the other options do not accurately describe immunotherapy. Organ function replacement pertains more to treatments for organ failure rather than cancer. The use of radioactive substances is specific to radiation therapy, which is a distinct treatment modality focused on directly damaging cancer cells through radiation exposure. Finally, the direct killing of cancer cells is characteristic of many traditional cytotoxic chemotherapies but does not encompass the broader and more nuanced approach of modulating immune responses like immunotherapy does.