Used in the treatment of autoimmune diseases such as systemic lupus erythematosus (SLE) and rheumatoid arthritis by neutralizing the activity of interleukin-21 (IL-21), a cytokine involved in B-cell and T-cell activation. It helps reduce autoantibody production, inflammation, and tissue damage. Also investigated in type 1 diabetes and other immune-mediated conditions to modulate pathogenic immune responses. Employed as a reference standard in preclinical and clinical studies to evaluate the efficacy and binding characteristics of anti-IL-21 therapeutics.