The German vaccine is a very clever way of getting the immune system to recognize and respond to your tumor. To understand it you need to understand something about the immune system. The immune system is an astoundingly complex and wonderful system - perhaps second only to the brain in complexity and mystery. But there's also a lot I don't understand. The immune system attacks things it recognizes as foreign.
Antigens are commonly proteins, or more specifically small parts of proteins.
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.