Bovine IL-21 (Interleukin-21) is a pleiotropic cytokine belonging to the common γ chain (γc) family that plays a central role in regulation of adaptive immunity in cattle (Bos taurus). 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 pathways such as JAK/STAT (predominantly STAT3) to modulate gene expression in target cells. IL-21 promotes B cell proliferation, differentiation, class-switch recombination, and antibody production, supports CD8⁺ T cell cytotoxic function, and influences NK cell activity, while also contributing to regulation of Th17 responses. In healthy cattle, basal IL-21 expression is low but increases during infection, vaccination, and immune activation. Bovine IL-21 has been implicated in immune responses to pathogens such as bovine viral diarrhea virus (BVDV), bovine leukemia virus (BLV), Mycobacterium bovis, and agents of bovine respiratory disease complex. In veterinary and agricultural research, IL-21 serves as a key biomarker of T cell–dependent B cell help and humoral immunity and is studied to evaluate vaccine efficacy, host–pathogen interactions, and adaptive immune regulation in cattle.