For years, doctors have given cancer patients three main treatments: surgery, radiation and chemotherapy. Now researchers are developing a fourth weapon: the patient's own immune system. New vaccines and drugs can stimulate the production of an army of cells and antibodies that kill cancer cells. Drug-vaccine therapy may be lifesaver for Deerfield man. Few people survive advanced melanoma, but immune therapy is giving Deerfield resident Douglas Parker a fighting chance. The 46-year-old salesman noticed a mole on his chest three and a half years ago that was found to be cancerous. Doctors removed the mole but didn't get all of the cancer. The cancer spread to other parts of his body, including his liver, where a tumor grew as large as a baseball. Parker took interferon and interleukin-2 to boost his immune system's ability to fight the cancer. The tumor shrank but didn't disappear. In August, 1997, surgeons removed it, along with two-thirds of his liver. Last January, doctors discovered a new tumor on Parker's left adrenal gland. He received an experimental cancer vaccine at the University of Chicago Hospitals, but the vaccine didn't stop the cancer from spreading to his right adrenal gland. To augment the vaccine, doctors at Lutheran General Hospital gave Parker a new round of interleukin-2 and interferon. The drug-vaccine combination has shrunk the tumors. And while it's too early to pronounce Parker cured, immune therapy may save his life. 'I want to do this to help myself as well as other people who have melanoma, ' he said. Immune therapy 'ultimately will be a significant change in the way we treat a lot of different cancers,' said Dr. Jon Richards of Lutheran General Hospital in Park Ridge, who is testing cancer vaccines on melanoma patients. 'It will be an equal partner with the other three treatments in the next five to ten years.' Several drugs that bolster the immune system have been approved, and vaccines are being tested in dozens of clinical trials, including several in the Chicago area. Many of the experimental vaccines have been tested on patients with advanced melanoma who have little chance of surviving with conventional treatments alone. Researchers also have begun doing work that could lead to vaccines to treat prostate, lung, colon and other cancers. Immune therapy alone won't cure cancer. But when used after conventional treatments, it could kill cancer cells that survive surgery, radiation or chemotherapy, researchers said. Some day, vaccines also might be able to prevent certain cancers. It may be possible to vaccinate against viruses and bacteria that help cause cervical, liver and stomach cancers, the National Cancer Institute said. The 'fourth weapon' cures cancer by______.