M2 macrophages, which contribute to the progression of ESCC by promoting tumor cell growth, invasion and metastasis, as well as restraining anti-tumor immune response cells [30], are characterized by the specific receptors known as CD163 (hemoglobin scavenger receptor) and CD204 (macrophage scavenger receptor I) [31,32]. Here, CD163 is linked to esophageal squamous cell carcinoma.