Th1 cells secrete gamma interferon (IFN-γ) and tumor necrosis factor alpha (TNF-α) and are important for resistance to intracellular pathogens and cancer, whereas Th2 cells secrete cytokines interleukin-4 (IL-4), IL-5, IL-9 and IL-13, which stimulate B-cell antibody production and resist other extracellular pathogens (30–32). Here, IL4 is linked to cancer.