The relationship between the immune system and cancer is incredibly complicated. There is, however, some fascinating research dealing with using pathogens that

The relationship between the immune system and cancer is incredibly complicated. There is, however, some fascinating research dealing with using pathogens that somehow preferentially infect cancer areas of the patient, to stimulate the immune system in that area: http://stm.sciencemag.org/content/2/44/44ra57.abstract http://stm.sciencemag.org/content/2/44/44ra57.abstract is a paper that talks about it. It's pretty neat!