The origins, differentiation and function characteristics of Tumor-associated macrophages (TAMs). TAMs can be developed from embryonic-derived tissue-specific resident macrophages and bone marrow-derived macrophages (BMDMs). As the ratio of cytokine types and concentrations in tumor microenvironment (TME) changes, the activation phenotype of TAMs changes dynamically to play different functional roles. In conventional binary model, macrophages polarize into two subtypes, M1-type and M2-type. In response to stimulation by different factors, M2-type TAMs further polarize to four phenotypes, including M2a, M2b, M2c and M2d. They secrete various factors that have significant impacts on the tumor progression.