When a virus infects a cell, it takes over the cells protein manufacturing equipment to make viral proteins which are assembled into new viruses. Tumor cells may also make abnormal proteins and can also be destroyed in this way. If the immune system can recognize and respond to an abnormal protein displayed on the tumor cell's surface, that is. Once cytotoxic T-cells are activated, they will not only kill cells displaying the antigen they recognize, but they will also multiply. Cytotoxic T-Cells are activated by encounters with special immune cells called Antigen Presenting Cells.
Dendritic cells are a type of antigen presenting cell.
When an antigen presenting cell eats something, it processes it and displays possible antigens on its surface in a way that is similar to how all cells display samples of what is inside them. It then becomes activated and is ready to actually destroy cells it encounters which display its specific antigen. The activated cells are also stimulated to multiply. Cytotoxic T-Cells normally reside in immune tissues like the lymph nodes. When APCs enter a lymph node, and an antigen is recognized, an army of specific killers is created which goes forth from the node to do battle with the enemy.