The immune system provides one of the body's main defenses against cancer. When normal cells turn into cancer cells, some of the antigens on their surface change. These new or altered antigens flag immune defenders, including cytotoxic T cells, natural killer cells, and macrophages. According to one theory, patrolling cells of the immune system provide continuing bodywide surveillance, spying out and eliminating cells that undergo malignant transformation.
Tumors develop when the surveillance system breaks down or is overwhelmed.
Some tumors may elude the immune defenses by hiding or disguising their tumor antigens. Furthermore, studies indicated that cancer patients have a better prognosis when their tumors are infiltrated with many immune cells. Immune responses may underlie the spontaneous disappearance of some cancers. Because such antigens develop not only in cancer but in other diseases as well, the antibody tests are not useful for cancer screening in the general population. They are however, valuable in monitoring the course of disease and the effectiveness of treatment in patients known to have cancer.