Key pro-inflammatory cytokines implicated in RA – interleukin-2 (IL-2), tumor necrosis factor-α (TNF-α), and IL-17 – are primarily produced by Th1 and Th17 cells, whereas Th2- and Treg-derived IL-4 and TGF-β exert protective, disease-limiting functions. Here, IL17A is linked to rheumatoid arthritis.