Feline GM-CSF (Granulocyte–Macrophage Colony-Stimulating Factor, CSF2) is a hematopoietic cytokine belonging to the colony-stimulating factor family, which also includes G-CSF (CSF3) and M-CSF (CSF1) that regulate the development, proliferation, and activation of myeloid immune cells. In cats (Felis catus), GM-CSF is produced by activated T lymphocytes, macrophages, endothelial cells, and fibroblasts in response to infection, inflammatory cytokines, or immune stimulation. Feline GM-CSF binds to the GM-CSF receptor (CSF2R) expressed on hematopoietic progenitor cells and mature myeloid cells, activating intracellular signaling pathways such as JAK/STAT, MAPK/ERK, and PI3K–AKT, which promote the proliferation and differentiation of granulocytes, macrophages, and dendritic cells, as well as enhance their functional activity in host defense. In feline immunity, GM-CSF plays an important role in regulating innate immune responses and antigen presentation, particularly during viral and bacterial infections. Cats are also valuable animal models for human immunological diseases, and GM-CSF–regulated myeloid responses have been studied in feline immunodeficiency virus (FIV) infection, a widely used animal model for human HIV/AIDS, where GM-CSF influences macrophage activation and dendritic cell function, helping researchers understand immune dysregulation and antiviral immune responses relevant to both veterinary and human medicine.