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 with various cytokines (e.g. anti-CD3 monoclonal antibody (CD3McAb), interleukin (IL)-2, interferon (IFN)-γ and IL-1α) in vitro for a period of time. The mechanism of action of CIK cells includes the release of various inflammatory cytokines such as IFN-γ, TNF-α, perforin, granzyme B, etc. to kill tumor cells, as well as the induction of apoptosis through the Fas pathway. -The proportion of CD3+CD56+ cells was significantly increased. CIK cells have unique advantages, including rapid proliferation, high tumoricidal activity, broad tumoricidal spectrum, sensitivity to multi-drug resistant tumor cells, tumoricidal activity independent of 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. In addition, our study showed that CD4+ T cells in CIK cells exerted their antitumor effects by upregulating the body’s 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. 2, CIK cells clinical applications At present, CIK cells are mainly used for the decontamination of autologous bone marrow grafts, the removal of small residual lesions and immunotherapy of advanced malignancies (including acute and chronic hematologic malignant diseases and various solid tumors). The efficacy of CIK cells in acute and chronic myeloid leukemia has been confirmed. In 111 acute leukemia patients treated with autologous CIK or DC-CIK, 96 patients achieved complete remission (CR) and a 7-year disease-free survival rate (DFS) of 66%, which was significantly higher than those treated with chemotherapy or autologous HSCT alone. The side effects of the treatment were mild and well tolerated by patients. Schmidt Wolf et al. treated 10 patients with metastatic solid tumors with human IL-2 cDNA-transfected CIK cells, with one CR and three stable cases. The side effects were mild and normalization could be achieved within a short period of time.