Even though a previous study reported increased IL‐21 messenger RNA or IL‐21 protein levels in serum, impaired tissues, and peripheral blood mononuclear cells from autoimmune diseases patients and its correlation with the initiation and development of diseases like RA,22 this is the first investigation to report the interplay between IL‐21 cytokine, vitamin 25(OH)D, and ROS in the progression of RA among patients. This evidence concerns the gene IL21 and autoimmune disease.