Equine IL-2 (Interleukin-2) is a central T cell–derived cytokine that regulates lymphocyte proliferation, differentiation, and immune homeostasis in horses (Equus caballus). 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 horses, basal IL-2 expression is generally low but increases during infection, vaccination, and immune stimulation. Altered IL-2 responses have been associated with equine herpesvirus (EHV) infection, equine influenza, strangles (Streptococcus equi), equine infectious anemia virus (EIAV), and immune-mediated inflammatory diseases. Horses serve as important natural models for viral respiratory disease, chronic inflammatory airway disease (equine asthma), and lentiviral infection (EIAV as a model for HIV). In veterinary and comparative medicine research, equine IL-2 is widely used as a biomarker of T cell activation and cell-mediated immunity and is studied to evaluate vaccine efficacy, host–pathogen interactions, and immune regulation in equine health.