IL21 and rheumatoid arthritis: In particular, we focus on cytokines that play central roles in RA pathogenesis across these stages, including IL-6 and TNF-α as major drivers of inflammation and joint damage, IL-1β and IL-17 as amplifiers of synovitis, IL-21 as a key mediator of T follicular helper cell (Tfh)/T peripheral helper cells (Tph)–B cell interactions, B cell-activating factor (BAFF) and A proliferation-inducing ligand (APRIL) as regulators of B cell survival and autoantibody production, and type I interferon (IFN) as a critical factor in the preclinical phase.