Atlantic salmon IL-21 (Interleukin-21) is a cytokine of the common γ chain (γc) family that plays an important role in regulation of adaptive immune responses in Atlantic salmon (Salmo salar). IL-21 is produced primarily by activated T lymphocytes, including CD4-like T 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—particularly STAT3—to regulate gene expression in target immune cells. In teleost fish, IL-21 promotes B cell proliferation, differentiation, and immunoglobulin production, and supports cytotoxic T cell responses, contributing to both humoral and cell-mediated immunity. In healthy Atlantic salmon, basal IL-21 expression is generally low but increases during infection, vaccination, and immune stimulation. IL-21 has been implicated in host responses to important aquaculture pathogens such as infectious salmon anemia virus (ISAV), piscine orthoreovirus (PRV), infectious pancreatic necrosis virus (IPNV), and bacterial infections including Aeromonas salmonicida. In fish immunology and aquaculture research, Atlantic salmon IL-21 serves as a biomarker of T cell–dependent adaptive immunity and vaccine responsiveness, providing insight into host–pathogen interactions and immune regulation in economically important marine species.