CD4 and cancer: According to the mechanisms of action of the cancer vaccines as well as the cancer immunity cycle discussed earlier, some of the key factors for a successful cancer vaccine design are the selection of the appropriate tumor antigen to stimulate effective T cells, the achievement of a sufficient antigen concentration in APCs in such a way as to activate them, as well as the inducement of durable immunogenic responses by activating the effector T cells, i.e., CD4+ and CD8+ [23].