**5. Immune system and cancer**

The relationship between the immune system and the tumor's ability to evade it is a hallmark of cancer [19]. An increasing number of studies have investigated the role of the immune system in tumor progression and its therapeutic potential for various tumors. The immune system comprises several cell types, and recognizes and eliminates biological, chemical, and physical dangers from the body via a series of humoral and cellular pathways and interactions [20]. In contrast, a neoplastic cell is an autologous cell that harbors suppression and overexpression of certain genes, mainly tumor suppressor genes and oncogenes, respectively. Neoplastic cells express proteins that are not recognized by the immune system; thus, receptors cannot indicate to the immune system that the cell is abnormal to the organism. Therefore, the immune system is an important factor in the body to prevent tumor progression. Taken together, it is necessary for the tumor to employ mechanisms to evade the antitumor immune response.

A neoplastic cell proliferates in an uncontrolled manner and each daughter cell may be the same as its mother cell; however, many cells accumulate new genetic defects, generating intratumor heterogeneity. This allows the neoplasm to adapt to different adversities for its progression, such as the antitumor action of the immune system. This process of interaction between neoplastic cells, immune cells, and the entire tumor microenvironment presents a complex cell-cell relationship. The most accepted "tumor immunoediting theory" that explains the events of this interaction is divided into the following three phases [21]:

