Elevated levels of IL-6, released by tumor-infiltrating lymphocytes, M2 phenotype macrophages [95], cancer-associated fibroblasts [96] and tumor cells [97,98], can activate a pro-tumorigenic IL-6/STAT3 pathway [99], inhibiting dendritic cell maturation, suppressing CD8+ T-cell and NK cell activation [100,101,102] and promoting CD4+ T cell differentiation to a T regulatory phenotype [103]. Here, STAT3 is linked to cancer.