Cancer cells have differences that make them "foreign" and that could be recognized by immune cells that not only protect us from infectious agents but eliminate many of the cells that have been modified.
When tumors develop, cells can be selected that make these differences less obvious or manage to turn off the immune response. The immune system is an ally cancer therapy, antibodies that recognize molecules on their surface are used and others have been introduced that prevent the tumor from sending off signals.
Genetically modified lymphocytes that kill transformed cells are now in use.
