The body has a very effective immune system, which is aimed at recognising and disposing of abnormalities in the body such as infections and cancer cells. A cancer can form and increase in size when its cells grow to outstrip the immune system, or to disguise themselves so that they are no longer seen as a threat by the immune system and are not therefore cleared.

Following clinical research, medications have been developed that can either boost the natural immune system (to try to make it more effective) or to make the cancer cells more visible again (to give the immune system a better chance to attack them). Another possible approach is to boost the immune system to a particular component of a cancer cell, which can enhance the immune system in a directed way at this cancer target.

Immunotherapy treatments are available in some, but not all cancers and they still can have side effects. These include the immune system overshooting and attacking normal body cells so that inflammation can occur in organs such as the lung, liver and bowel, which can be severe. Hormone producing organs such as the thyroid gland can also be affected, so careful monitoring is required during these treatments.