**6. Tumor immune evasion mechanisms**

The immune system can react with agents that are harmful to the body in several ways. Innate immunity, responsible for this first interaction, can corroborate the antitumor activity with a nonspecific reaction to the tumor [22], such as treatment with Bacillus de Calmette and Guerin of human melanomas [23], that initially induces a nonspecific response to this agent, but later affects tumor progression. In contrast, innate immunity, which is primarily responsible for the antitumor activity of the immune system, is triggered by recognition of tumor cells by CD8+ T cells. They eliminate tumor cells by various pathways, such as perforin/granzyme or by induction of apoptotic pathways [22].

The interaction between the immune system, neoplastic cells, and tumor microenvironment is extremely complex and orchestrated by numerous regulatory factors, whether intrinsic or extrinsic to neoplastic cells. Neoplastic cells employ several tactics for evading the immune system, and consequently, the permanence of the escape phase in the immunoediting process. Cells use several mechanisms to succeed in this process, ranging from the non-recognition of cells as non-self to the production of inhibitory factors and exhaustion of immune system cells, including the activation of bone marrow-derived suppressor cells, activation of regulatory T cells (Tregs), alteration of dendritic cell functions, production of cytokines, and non-recognition of cells due to non-expression of histocompatibility molecules [24].
