Rabbit IL-21 (Interleukin-21) is a cytokine of the common γ chain (γc) family that plays a central role in regulating adaptive immune responses in rabbits (Oryctolagus cuniculus). IL-21 is produced primarily by activated CD4⁺ T cells, particularly T follicular helper (Tfh) cells and Th17 cells, following antigen stimulation. It signals through the IL-21 receptor (IL-21R) in association with the common γ chain, activating downstream JAK/STAT pathways—predominantly STAT3—to regulate gene expression in B cells, T cells, and natural killer (NK) cells. IL-21 promotes B cell proliferation, plasma cell differentiation, immunoglobulin class switching, and antibody production, while enhancing CD8⁺ T cell cytotoxic function and NK cell activity. In healthy rabbits, basal IL-21 expression is generally low but increases during infection, vaccination, and immune activation. Rabbit IL-21 is particularly relevant in studies of vaccine immunogenicity, tuberculosis models, atherosclerosis research, ocular inflammation, and tumor immunology, where robust T cell–dependent antibody and cytotoxic responses are important. In biomedical research, rabbit IL-21 serves as a biomarker of T cell–dependent B cell help and humoral immunity and supports investigation of adaptive immune regulation and immunotherapeutic strategies.