In fact, cancer-derived soluble factors can regulate the development, accumulation, and functions of several tumor-infiltrating leukocytes such as tumor-associated macrophage (TAM), M2 macrophage, T-helper 1 (Th1), follicular helper T (Tfh), Tregs, and DCs by switching on a STAT3-dependent signaling (70). Here, STAT3 is linked to cancer.