Proposed Mechanism of Action The vaccines are created by incorporating tumor-associated antigens (TAA) and costimulatory molecules into a vaccine vector. Injecting the vaccine into the patient triggers the immune system. Antigen-presenting cells absorb the vaccine and display the antigens and costimulatory molecules to helper T cells. The helper T cells then release chemicals, called cytokines, which activate another class of T cells known as cytotoxic or killer T cells. These T cells now recognize, attack, and destroy cancer cells that bear these antigens.