The power of the immune system derives from its ability to recognize a wide array of pathogens as foreign.
Cancer is an internal pathogen.
Cancer cells are genetically different from the normal cells from which they derive. The immune system recognizes cancer cells as foreign because of the genetic changes that occur during malignant transformation. Vaccinating patients with their own malignant cells forces the immune system to recognize the cancer as foreign. Vaccination by itself does not stimulate the immune system to reject cancer. However, once vaccination has forced the immune system to recognize the cancer, immune lymphocytes can be obtained and activated outside the body to kill malignant cells.