Human CCL1 (C-C motif chemokine ligand 1), also known as I-309, is a member of the CC chemokine family that plays a role in regulating leukocyte trafficking and immune responses. CCL1 is produced primarily by activated CD4⁺ T lymphocytes, monocytes, macrophages, dendritic cells, and certain endothelial and epithelial cells in response to inflammatory cytokines and immune stimulation. It signals predominantly through the CCR8 receptor to promote chemotaxis and activation of T cells, monocytes, and regulatory T cells, contributing to the orchestration of immune responses at sites of inflammation. In healthy individuals, circulating CCL1 levels are typically low, but expression increases during infection, allergic inflammation, autoimmune disease, and tumor-associated immune responses. Elevated CCL1 has been implicated in conditions such as asthma, atopic dermatitis, inflammatory bowel disease, rheumatoid arthritis, and certain cancers, where it may contribute to chronic inflammation or immune cell recruitment within the tumor microenvironment. In clinical and translational research, human CCL1 serves as a biomarker of immune activation and Th2- or regulatory-associated responses and represents a potential therapeutic target for modulating inflammatory and immune-mediated diseases.