Preventive vaccines are administered to healthy people to protect them against future infection. A vaccine typically contains an agent that resembles a disease-causing micro organism, and is often made from weakened, or killed forms, or parts of the microbe. The agent stimulates the body's immune system to recognize the agent as foreign, destroy it, and "remember" it, so that the immune system can more easily recognize and destroy any of these micro organisms that it encounters later.
Preventive vaccines generally induce a humoral immune response whereby antibodies are produced and accessory processes are activated that lead to pathogen elimination before it can multiply extensively. The foreign antigen is generally neutralized by circulating antibodies, before it enters a host cell, or other protected niche, and is subsequently eliminated by classical complement activation, opsonin promotion and phagocytosis (uptake of the microbe and its destruction by a phagocyte).
Therapeutic vaccines differ from preventive vaccines in their ability to induce cell-mediated immunity rather than antibody- and complement-mediated immunity. This characteristic helps the host to fight pathogens and infection microbes once they have already entered cells or entered other protected niches in the body.
Cell-mediated immunity is the immune response to an antigen of an agent whereby certain specific immunocompetent cells are activated: macrophages, natural killer cells (NK) and T cells. There are many different types of T cells, of which at least two are very important for an effective immune response: antigen-specific cytotoxic CD8+ T cells and helper CD4+ T cells also called helper T cells. Similarly to the humoral immune response where certain ‘B’ cells are responsible for memory, cell-mediated immunity will induce specialized ‘T’ cells to “remember” the antigen. T helper and Killer cells play an important role in this process.
Good reviews on therapeutic vaccines against cancer and infectious diseases are given by Ward S. & Dalgleish A. 2007 and S-J. Ha et al., 2008; Huh WK & Roden RB, 2008, Hung CF et al. 2008 and Trimble CL & Frazer IH, 2009.