Human CX3CL1 (C-X3-C motif chemokine ligand 1), also known as fractalkine, is a chemokine belonging to the CX3C chemokine family, characterized by three amino acids separating the first two conserved cysteine residues. CX3CL1 is unique among chemokines because it exists in both a membrane-bound form and a soluble form, allowing it to function in leukocyte adhesion and chemotaxis. In humans (Homo sapiens), CX3CL1 is produced primarily by endothelial cells, epithelial cells, neurons, and macrophages in response to inflammatory cytokines, infection, or tissue injury. CX3CL1 binds to its receptor CX3CR1, which is expressed on monocytes, macrophages, natural killer (NK) cells, cytotoxic T lymphocytes, and microglia, promoting adhesion, migration, and activation of these immune cells at sites of inflammation. In human physiology, the CX3CL1–CX3CR1 signaling pathway plays important roles in immune surveillance, neuroimmune communication, and vascular inflammation. Dysregulation of this pathway has been implicated in chronic inflammatory diseases, neurodegenerative disorders such as Alzheimer’s disease, and cardiovascular diseases including atherosclerosis, where fractalkine-mediated monocyte recruitment contributes to vascular plaque formation. Because of its role in leukocyte trafficking and inflammatory signaling, human CX3CL1 is widely studied in immunology, neurobiology, and cardiovascular disease research, and is considered a potential therapeutic target for inflammatory and neurodegenerative diseases.