Equine IL-18 (Interleukin-18) is a pro-inflammatory cytokine belonging to the IL-1 family that plays a central role in innate and adaptive immune responses in horses (Equus caballus). IL-18 is produced primarily by monocytes, macrophages, dendritic cells, and epithelial cells as an inactive precursor that requires cleavage by caspase-1 following inflammasome activation to become biologically active. Once activated, IL-18 stimulates production of interferon-γ (IFN-γ) by T cells and natural killer (NK) cells, thereby promoting Th1-type immune responses and enhancing cell-mediated immunity against intracellular pathogens. In healthy horses, basal IL-18 expression is generally low, but levels increase during bacterial and viral infections, systemic inflammatory responses, and immune stimulation. Elevated IL-18 has been implicated in equine infectious diseases such as equine herpesvirus (EHV) infection, strangles (Streptococcus equi), and other inflammatory conditions. In veterinary and comparative medicine research, equine IL-18 serves as a biomarker of inflammasome activation and Th1-mediated immune activation, providing insight into host–pathogen interactions, vaccine responses, and inflammatory disease mechanisms in equine health.