The immune system is a complex network of organs, tissues, and specialized cells. It recognizes and destroys foreign invaders, such as bacteria or viruses, as well as some damaged, diseased, or abnormal cells in the body, including cancer cells. An immune response is triggered when the immune system encounters a substance, called an antigen, it recognizes as “foreign.”

White blood cells are the primary players in immune system responses. Some white blood cells, including macrophages and natural killer cells, patrol the body, seeking out foreign invaders and diseased, damaged, or dead cells. These white blood cells provide a general—or nonspecific—level of immune protection.

Other white blood cells, including cytotoxic T cells and B cells, act against specific targets. Cytotoxic T cells release chemicals that can directly destroy microbes or abnormal cells. B cells make antibodies that latch onto foreign intruders or abnormal cells and tag them for destruction by another component of the immune system. Still other white blood cells, including dendritic cells, play supporting roles to ensure that cytotoxic T cells and B cells do their jobs effectively.

It is generally believed that the immune system’s natural capacity to detect and destroy abnormal cells prevents the development of many cancers. Nevertheless, some cancer cells are able to evade detection by using one or more strategies. For example, cancer cells can undergo genetic changes that lead to the loss of cancer-associated antigens, making them less “visible” to the immune system. They may also use several different mechanisms to suppress immune responses or to avoid being killed by cytotoxic T cells.

The goal of immunotherapy for cancer is to overcome these barriers to an effective anticancer immune response. These biological therapies restore or increase the activities of specific immune-system components or counteract immunosuppressive signals produced by cancer cells.