Within the tumor microenvironment, CD4+ T cell-derived cytokines such as interferon-gamma (IFN-γ) contribute to the recruitment and homing of immune cells to the tumor site, while interleukins such as IL-2 and IL-21 are essential for the differentiation and maintenance of high-affinity cytotoxic CD8+ T cells [79,86]. Here, IL21 is linked to neoplasm.