This animation from Nature illustrates how tumor cells are sensed and destroyed by the immune system and how tumors evolve and detect immune-mediated eliminations.
Ultraviolet light from the sun, for example, can cause chronic damage to skin cells creating melanoma. The immune system continuously monitors for this kind of damage. Immune cells known as natural killer cells send stress signals to recruit more cells involved in the immune reaction. Active natural killer cells release molecules to cause apoptosis in damaged cells.
Tumors cells can evolve genetically to evade the immune reaction. Genetic changes may cause the tumor to no longer express the once recognized marker used by the immune system to find and destroy. This evolution can allow the tumor to grow undetected.
Immunotherapies like adaptive T cell transfer, will select a patient's cytotoxic T cells best fit for tumor clearance and infuse them back into a patient for aggressive results. This immunotherapy and other methods like it can be used to treat several different cancer types.