Flying Fox Bat (Pteropus spp.) Interleukin-4 (IL-4) is a Th2-associated cytokine produced primarily by activated CD4⁺ T helper 2 (Th2) cells and other type 2 immune populations, where it plays a central role in regulating humoral immunity, alternative macrophage activation, and modulation of inflammatory responses. IL-4 signals through receptor complexes containing IL-4Rα paired with either the common gamma chain (γc) or IL-13Rα1, activating JAK/STAT6 pathways that promote B-cell proliferation, antibody class switching, and suppression of certain pro-inflammatory cytokines. In flying fox bats—natural reservoir hosts for high-consequence zoonotic viruses such as Nipah virus and Hendra virus—IL-4 is of particular interest for understanding how Th2-type immune polarization contributes to immune balance and viral tolerance. Regulation of IL-4 signaling may help modulate Th1-driven antiviral responses, limit immunopathology, and support tissue repair during infection, potentially contributing to the ability of Pteropus species to harbor viruses with minimal clinical disease. Characterization of Flying Fox Bat IL-4 supports research into bat-specific immune regulation, host–virus co-evolution, and mechanisms of immune tolerance in reservoir species. Studying IL-4 dynamics in these bats enhances comparative immunology efforts and provides insight into conserved Th2 pathways relevant to allergy, chronic inflammation, and immune homeostasis in humans.