**Abstract**

Chronic inflammation can trigger events that would induce the malignant transformation of cells and carcinogenesis. Cytokines play a crucial role and can control the development and multiplication of cancerous cells. However, clinical data present controversy about the participation of these proteins in the establishment and development of cancer. Interleukin 10 (IL-10), a potent anti-inflammatory cytokine, has been the subject of multiple studies. Several studies have reported that IL-10 has pro- and antitumor effects. Elevated levels of IL-10 are associated with increased tumor growth with poor prognosis and drug resistance. However, this cytokine has both tumor-promoting and tumor-inhibiting properties. *In vitro* and *in vivo* studies report mechanisms by which IL-10 expression downregulates class I, which results in the control of the metastatic disease. IL-10 also inhibits tumorigenesis via downregulation of other cytokines. The variation observed could be the result of concentration ranges of this protein, genetic polymorphism, or both. The value obtained may serve as a biomarker indicative of tumor development and its prognosis.

**Keywords:** interleukin 10, IL-10, IL10 polymorphism, cancer biomarker

### **1. Introduction**

The immune system is responsible for monitoring and getting rid of molecules or cells outside the body that can be potentially malignant. This system is constituted by cells that can act by themselves or the synthesis of molecules capable of inducing the destruction of strange agents or invading cells. In addition, these cells synthesize proteins called cytokines, which are not only capable of destroying invading cells; they activate other cells of the same immune system, which makes the system efficient. In a coordinated way, the synthesis of anti-inflammatory cytokines begins that will stop the inflammatory process and prevent the damage from spreading in healthy tissue. In the case of cancer, the cells will have nonnormal characteristics due to the changes they undergo during their transformation. The immune system works to eliminate these cells; however, for reasons not well established yet, at some point, the immune system fails, and the malignant cells survive and establish [1]. Even when the production of anti-inflammatory cytokines is present, the levels of pro- and anti-inflammatory cytokines have an imbalance that prevents completing the final objective. An increase of anti-inflammatory cytokines, such as interleukin 10 (IL-10), has been associated with a poor prognosis and considered as a biomarker for cancer disease. This chapter presents evidence that shows alterations in the serum levels of IL-10 in cancer patients, the participation of the cancerous tumor in the synthesis of this cytokine, and its possible relationship with polymorphisms in the gene promoter, the gene of this protein.
