Human 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 humans (Homo sapiens), GM-CSF is produced by activated T lymphocytes, macrophages, endothelial cells, fibroblasts, and epithelial cells in response to infection, inflammatory cytokines, or immune stimulation. Human 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, and enhance their antigen-presenting and inflammatory functions. In human health, GM-CSF plays an important role in host defense against infections, regulation of inflammatory responses, and development of immune cells involved in adaptive immunity. Dysregulation of GM-CSF signaling has been associated with inflammatory and autoimmune diseases, including rheumatoid arthritis and pulmonary alveolar proteinosis. Because of its ability to stimulate immune cell development and activation, GM-CSF is widely studied and used in clinical immunotherapy and vaccine research, including GM-CSF–based cancer vaccines designed to enhance dendritic cell activation and anti-tumor immune responses.