Fig 2. Induction of an immune response by antigen vaccination. Active immunization occurs following administration of tumor antigens, which are processed by antigen-presenting cells resulting in activation of immune effector cells such as T lymphocytes and B lymphocytes. Effector cells fight the tumor through several different effector pathways such as antibodies, cytokines or direct cellular interaction (Fas/Fas ligand, perforin/granzymes). Ideally this results in immunologic memory with long-lasting immunity against the tumor. Stimulation of the innate immune system is also known to have an important role in the evolution of an adaptive immune response (e.g. a rapid burst of inflammatory cytokines leads to activation of DC and macrophages). However, tumor-specific T cells also secrete chemokines (e.g. RANTES, MIP-1) that attract cells of the innate immune system to the tumor site. This mechanism may further contribute to the tumoricidal activitiy exhibited during T cell-mediated tumor regression. (TCR: T cell receptor, MHC: major histocompatibility complex, DC: dendritic cell, PMN: polymorphonuclear neutrophil, NK: natural killer cell, NKT: NK T cell, RANTES: regulated upon activation, normal T cell expressed, and secreted, MIP-1: macrophage inflammatory protein 1).