Ultraviolet (UV) light is part of normal sunlight and has many effects on human skin and health. One of the harmful effects of long-term UV light exposure is that it can cause skin cancer. The mechanism by which UV light causes skin cancer is not entirely understood. One of the ways UV light causes cancer is by modifying DNA molecules in the cells of the skin. Another mechanism involved in cancer formation by UV light is immunosuppression. By this mechanism, UV light inactivates cells of the immune system of the skin. The immune cells are responsible for the detection and destruction of foreign substances and organisms such as bacterias and viruses but they also recognize and destroy cancer cells. UV light is known to prevent cells of the immune system to destroy cancer cells.