|
Seborrhoeic warts appear on the skin. This is caused by excessive exposure of skin to sun rays. If not treated in time, leads to skin cancer. It can be removed surgically.
UV-B is a complete carcinogen, meaning that it can function as both an initiator and a promoter, leading to tumor induction.
Exposure to a tumor initiator is believed to be a necessary but not sufficient step in the malignant process, since initiated skin cells not exposed to tumor promoters do not generally develop tumors. The second stage in tumor development is promotion, a multistep process whereby initiated cells are exposed to chemical and physical agents that evoke epigenetic changes that culminate in the clonal expansion of initiated cells and cause the development, over a period of weeks to months, of benign growths known as papillomas.
The relationship of sun exposure to melanoma is less clear-cut, but suggestive evidence supports an association. Melanomas occasionally develop by the teenage years, indicating that the latent period for tumor growth is less than that of nonmelanoma skin cancer.
It is known that UV-induced tumors in murine skin are antigenic and are rapidly rejected when transplanted into normal syngeneic animals. If the tumors are transplanted into animals previously exposed to subcarcinogenic doses of UV-B, they are not rejected and instead grow progressively in the recipients. This failure of irradiated animals to reject the transplanted tumors is due to the development of T suppressor cells that prevent the rejection response. While the mechanism of suppression of tumor rejection is unknown, such a response might be a critical determinant of cancer risk in human skin.
|
