Mallard (Anas platyrhynchos) Interleukin-10 (IL-10) is a key anti-inflammatory cytokine produced by regulatory T cells, macrophages, dendritic cells, and other immune cell populations, where it plays a central role in limiting excessive immune activation and maintaining immune homeostasis. IL-10 signals through the IL-10 receptor complex (IL-10R1/IL-10R2), activating JAK/STAT3 pathways that suppress pro-inflammatory cytokine production (including IL-1β, IL-6, and IFN-γ), reduce antigen presentation, and downregulate Th1-type immune responses. In mallards, which are natural reservoir hosts for avian influenza viruses (AIV), IL-10 is particularly important for balancing antiviral responses with minimal clinical disease, contributing to tolerance of viral infection while limiting tissue-damaging inflammation in respiratory and gastrointestinal tissues. IL-10 is also induced during infections with Salmonella, Campylobacter, and other waterborne pathogens, influencing pathogen persistence and transmission dynamics. As both a regulator of immune tolerance and a biomarker of anti-inflammatory signaling, mallard IL-10 is central to studies of avian influenza ecology, host–pathogen co-evolution, and reservoir host immunology. Characterization of IL-10 in this species enhances understanding of immune regulation in wild bird populations and provides comparative insight into mechanisms of viral tolerance and inflammatory control relevant to both veterinary and human infectious disease research.