Ferret IL-2 (Interleukin-2) is a central T cell–derived cytokine that regulates lymphocyte proliferation, differentiation, and immune homeostasis in ferrets (Mustela putorius furo). IL-2 is produced primarily by activated CD4⁺ and CD8⁺ T lymphocytes following antigen recognition and costimulatory signaling, and it functions through the IL-2 receptor complex (IL-2Rα/CD25, IL-2Rβ, and the common γ chain) to promote clonal expansion of antigen-specific T cells, enhance cytotoxic T lymphocyte and natural killer (NK) cell activity, and support development and maintenance of regulatory T cells (Tregs). In healthy ferrets, basal IL-2 expression is typically low but increases during infection, vaccination, and immune stimulation. Altered IL-2 responses have been observed in viral infections such as influenza and SARS-CoV-2, as well as in inflammatory and immune-mediated conditions. Ferrets serve as important translational animal models for human respiratory viral diseases, including influenza and COVID-19, due to similarities in lung physiology and viral pathogenesis. In biomedical research, ferret IL-2 is used as a biomarker of T cell activation and cell-mediated immunity, supporting studies of vaccine efficacy, antiviral immunity, and immunopathology in respiratory disease models.