In recent years, there have been new developments in cellular immunotherapy of tumors, and new approaches and theories are constantly being fleshed out. First of all, it is worth mentioning cytokine-induced killer (CIK) cells that emerged in 1991 as a new hope for tumor over-the-top immunotherapy. Currently, CIK cells are mainly used for decontamination of autologous bone marrow grafts, removal of microscopic residual lesions and immunotherapy of advanced malignancies (including acute and chronic hematologic malignancies and various solid tumors) with mild side effects and good patient tolerance. Therefore, CIK cell immunotherapy is worthy of further detailed and in-depth study. 1.What are CIK cells? CIK cells are a heterogeneous group of cells with non-major histocompatibility antigen (MHC)-restricted tumor-killing activity obtained by co-culturing human peripheral blood mononuclear cells in vitro with various cytokines [such as anti-CD3 monoclonal antibody (CD3McAb), interleukin (IL)-2, interferon (IFN)-γ and IL-1α, etc.] for a period of time. CIK cells have unique advantages, including rapid proliferation, high tumoricidal activity, broad tumoricidal spectrum, sensitivity to multi-drug resistant tumor cells, tumoricidal activity unaffected by immunosuppressive agents such as cyclosporine A (CsA) and FK506, low toxicity to normal bone marrow hematopoietic precursor cells, and resistance to tumor cell-triggered apoptosis of effector cells Fas-FasL. The mechanism of action of CIK cells includes the release of various inflammatory cytokines such as IFN-γ, TNF-α, perforin and granzyme B to kill tumor cells and the induction of apoptosis through the Fas pathway (Figure 1), and the proportion of CD3+CD56+ cells, the main effector cells of CIK cells exercising non-MHC-restricted killing activity, was significantly increased after CIK cell expansion. In addition, our study showed that CD4+ T cells in CIK cells exerted their antitumor effects by upregulating body immunity and inducing apoptosis of tumor cells through the release of various immunoreactive cytokines. We also demonstrated that co-incubation of CIK cells with dendritic cells (DCs) effectively reduced the proportion of suppressive CD4+CD25+ regulatory T cells, which significantly enhanced the killing activity of CIK cells against a variety of tumor cells. Biological Cell Immunotherapy Biological cell immunotherapy is a treatment method that uses anti-tumor cells with powerful cancer-killing function to kill tumor lesions and cancer cells, so as to treat tumors and control recurrence, metastasis and growth, with significant effects, even better than radiotherapy, and without toxic side effects. 1. Therapeutic advantages of biological cellular immunotherapy: (1) Exact effect and high efficiency. For some cancers, the efficiency is as high as 70%. (2) No toxic side effects of radiotherapy and chemotherapy, no pain for patients, good tolerance and strong tumor killing specificity. (3) It can stimulate the systemic anti-cancer effect and is equally effective for multiple foci or metastatic malignant tumors. (4) It can help the body to quickly restore the anti-cancer immune system destroyed by radiotherapy and chemotherapy and improve the long-term anti-cancer ability. (5) It is effective in preventing recurrence of cancer after surgery and has good long-term anti-cancer effect. (6) It can be used alone or in combination with other treatment methods. It is effective for single use and better for multiple use. Theoretically speaking, every tumor patient can benefit from biological cell immunotherapy. (2) The effect of prevention of recurrence after surgery of early stage tumor is remarkable. (3) It is effective in curbing tumor progression, prolonging patient’s life and improving quality of life for patients with intermediate and late stage tumors. (4) Treatment of multi-drug resistant tumor patients with ineffective radiotherapy and chemotherapy. (5) To help patients with weak constitution after receiving radiotherapy and chemotherapy to restore the function of the body’s anti-cancer immune system.