Used to measure interleukin-21 (IL-21) levels in human biological samples such as serum, plasma, or cell culture supernatants. It is commonly applied in immunology research to study immune responses, particularly in conditions involving T cells, B cells, and natural killer (NK) cells. The kit helps investigate the role of IL-21 in autoimmune diseases, viral infections, cancer immunotherapy, and inflammatory disorders. It enables researchers to quantify IL-21 with high sensitivity and specificity, supporting both clinical studies and drug development.