Th17 cells, a subset of T cells, secrete various pro-inflammatory cytokines, including IL-17, TNF-α, granulocyte-macrophage colony-stimulating factor (GM-CSF), interleukin-21 (IL-21), and interleukin-22 (IL-22), and are seemingly key mediators in RA pathogenesis [23,24]. Here, IL21 is linked to rheumatoid arthritis.