The immune system plays an essential role in the tumor progression; not only can it inhibit tumor growth but it can also promote tumor growth by establishing a favorable environment. Tumor cells utilize several strategies to evade the host's immune system, including expression of immunosuppressive molecules such as PD-L1, IDO and siglec-9. In addition, tumor cells not only regulate the recruitment and development of immunosuppressive forces to influence the tumor microenvironment but also shift the phenotype and function of normal immune cells from a possibly anti-tumor state to a pro-tumor state. As a result, tumor cells evade the host's immune system, leading to metastasis and/or recurrent disease.