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Human interleukin-21 receptor （IL21R）, also known as CD360 or NILR, is a type I cytokine receptor belonging to the type 4 subfamily. It contains two fibronectin type-III domains and is selectively expressed in lymphoid tissues, with the highest expression in the thymus and spleen. IL21R is crucial for the proliferation and differentiation of T cells, B cells, and natural killer （NK） cells, as it transduces the growth-promoting signals of interleukin-21 （IL-21）.