Susceptibility to cancer varies greatly. The Epstein-Barr virus that causes Burkitt's lymphoma in some individuals (mainly in Africa), for example, can also be found in healthy people throughout the world. Most often the virus is harmless; in some cases, it causes mononucleosis (involving a limited proliferation of white blood cells). Only rarely does this virus cause the uncontrolled proliferation of leukocytes characteristic of Burkitt's lymphoma. The reasons for these differences in response to the Epstein-Barr virus, and indeed for the differing susceptibilities of people to other forms of cancer, are not well understood.
It is known that cancer risk increases with age. According to one theory, this is due to the fact that aging lymphocytes gradually accumulate genetic errors that decrease their effectiveness. The functions of the thymus also decline with age in parallel with a decrease in cell-mediated immune competence. Both of these changes, and perhaps others not yet discovered, could increase susceptibility to cancer.
Numerous experiments have demonstrated that tumors grow faster in experimental animals subjected to stress than they do in unstressed control animals. This is generally attributed to the fact that stressed animals, including humans, exhibit increased secretion of corticosteroid hormones that act to suppress the immune system (which is why cortisone is given to people who receive organ transplants and to people with chronic inflammatory diseases). Some recent experiments, however, suggest that the stress-induced suppression of the immune system may also be due to other factors that do not involve the adrenal cortex. Future advances in cancer therapy may incorporate methods of strengthening the immune system into protocols aimed at directly destroying tumors.
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