IL21 and rheumatoid arthritis: The synovial fluid of RA patients has been found to contain high levels of IL-12 [45], IL-18 [46], IL-15 [47], IL-7 [48] and IL-21 [49], and cytokines such as IL-12, IL-18 and IFN-α are produced by activated DCs and macrophages and have also been implicated in chronic inflammation [30], making it plausible that NK cells, in a chronic inflammatory setting, can become activated by several cytokines.