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IL-21 belongs to the IL-15/IL-21 family. It is a cytokine with immunoregulatory activity. IL-21 is a T-cell–derived cytokine that works in concert with other γc cytokines. It synergizes with IL-7 and IL-15 to expand and activate CD8 T cells. IL-21 also augments the activity of NK cells. IL-21, along with IL-6, drives differentiation of Tfh. Tfh cells are found preferentially in B-cell follicles, where, under the control of the transcription factor BCL6, they regulate B-cell development, activation, and class switching. Tfh cells are also a source of IL-21. IL-21 appears to have some anticancer properties, and it has been tested in the treatment of melanoma.