Ovine/Caprine Interleukin-12/Interleukin-23 p40 (IL-12/IL-23 p40) is a cytokine subunit that plays an important role in regulating immune responses in sheep (Ovis aries) and goats (Capra hircus). The IL-12/IL-23 p40 protein is identical at the amino acid level in these two species, reflecting strong conservation of this immune signaling molecule in small ruminants. 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. Ovine and caprine IL-12/IL-23 p40 contributes to activation of natural killer (NK) cells and differentiation of T helper cells toward Th1-type immune responses, promoting production of interferon-γ (IFN-γ) and enhancing cell-mediated immunity against intracellular pathogens. Through IL-23 signaling, the p40 subunit also supports maintenance and expansion of Th17 cells involved in inflammatory responses and host defense. In sheep and goats, IL-12/IL-23 p40 participates in immune responses against bacterial, viral, and parasitic infections, including important livestock pathogens such as Mycobacterium avium subsp. paratuberculosis, Brucella spp., and gastrointestinal nematodes. Due to its role in regulating cellular immunity, ovine/caprine IL-12/IL-23 p40 is widely studied in veterinary immunology and translational research, including investigations of host–pathogen interactions, vaccine development, and immune responses relevant to small ruminant health and comparative biomedical research.