Bovine interleukin-3 (IL-3) is a hematopoietic growth factor produced primarily by activated T lymphocytes and mast cells that regulates the proliferation, differentiation, and survival of multipotent hematopoietic progenitor cells through signaling via the IL-3 receptor complex (IL-3Rα/CD123 and the common β chain), activating JAK/STAT, MAPK, and PI3K pathways. In cattle, IL-3 supports the development and expansion of myeloid lineage cells—including monocytes, macrophages, dendritic cells, basophils, eosinophils, and mast cells—thereby influencing innate immune responses, inflammatory regulation, and host defense. In bovine health, IL-3 is relevant to immune responses during respiratory diseases (such as bovine respiratory disease complex), mastitis, parasitic infections, and allergic or hypersensitivity conditions that can impact productivity and herd performance. Dysregulated IL-3 signaling may contribute to excessive inflammation or altered myeloid cell dynamics during infection. Beyond its role in livestock health, bovine IL-3 is valuable in comparative immunology and large-animal research models, supporting studies of hematopoiesis, inflammatory disease mechanisms, and evaluation of immunomodulatory therapeutics relevant to both veterinary and human medicine.