Feline Interleukin-12/Interleukin-23 p40 (IL-12/IL-23 p40) is a cytokine subunit that plays an important role in regulating immune responses in cats (Felis catus). The p40 subunit is shared by two heterodimeric cytokines, IL-12 and IL-23, where it pairs with the p35 subunit to form IL-12 and with the p19 subunit to form IL-23. These cytokines are produced primarily by activated antigen-presenting cells, including macrophages and dendritic cells, in response to microbial infection or inflammatory stimuli. Feline IL-12/IL-23 p40 contributes to activation and differentiation of T helper cells and natural killer (NK) cells, promoting Th1-type immune responses and production of interferon-γ (IFN-γ). Through IL-23 signaling, the p40 subunit also supports the maintenance and expansion of Th17 cells involved in inflammatory responses and host defense. In cats, IL-12/IL-23 p40 participates in immune responses against intracellular pathogens, viruses, and other infectious agents and has been studied in the context of feline infectious diseases such as feline immunodeficiency virus (FIV) and feline infectious peritonitis (FIP). Due to similarities between feline and human immune pathways, feline IL-12/IL-23 p40 is also relevant in translational research, where feline models are used to investigate mechanisms of immune regulation, chronic inflammation, and cytokine-driven disease, as well as to evaluate immunomodulatory therapies and vaccine strategies that may inform both veterinary and human medicine.