5 Immune system: a double-edged sword in cancer

The objective of the review is to examine the role of innate and adaptive immune cells in cancer. Immune system functions as a host defensive mechanism protecting against invading pathogens and transformed cells, including cancer. However, a body of research carried out over the last few decades has disclosed the unexpected role of immune system in fostering the tumor growth. A computer-based online search was performed in the PubMed, Scopus and Web of Science databases for articles published, concerning natural killer (NK) cells, Macrophages, CD4+ and CD8+ T cells with relevance to cancer. After finding relevant articles within these search limits, a manual search was conducted through the references from these articles. This review summarizes the role of immune system in Immunosurveillance and Immunoediting. It then focused mainly on role of macrophages, regulatory T cells (Treg), T(H)17 cells and on the immunosuppressive mechanisms, which facilitate immune evasion of tumor cells. Our results shows that, immune cells, such as CD8+ cytotoxic T lymphocytes (CTL), CD4+ T helper (T-H)1 cells and NK cells along with their characteristic cytokine interferon (IFN)-gamma, function as major antitumor effector cells. Whereas CD4+T(H)2 cells, myeloid-derived suppressor cells (MDSCs) and their derived cytokines function as dominant tumor-promoting forces. In contrast to these cells, macrophages, Treg, and T(H)17 cells show a dual effect in cancer. Thus, it appears that most components of the immune system are potentially endowed with dual functions i.e., promoting tumor development on the one hand and restraining tumor development on the other and hence immune system can be considered as a double-edged sword in cancer.