Human CCL18 (C-C motif chemokine ligand 18), also known as pulmonary and activation-regulated chemokine (PARC), is a member of the CC chemokine family that plays a significant role in immune regulation, particularly in Th2-associated and fibrotic conditions. CCL18 is produced predominantly by alternatively activated (M2) macrophages and dendritic cells, especially within the lungs and lymphoid tissues. It functions as a chemoattractant for naïve T cells, B cells, and immature dendritic cells, and has been reported to interact with receptors including CCR8 and PITPNM3, contributing to immune cell recruitment and tissue remodeling. In healthy individuals, circulating CCL18 levels are detectable at low to moderate concentrations, but levels increase substantially in chronic inflammatory and fibrotic diseases. Elevated CCL18 has been strongly associated with pulmonary fibrosis, systemic sclerosis, atopic dermatitis, asthma, rheumatoid arthritis, and certain malignancies, where it may contribute to fibrosis, tumor progression, or maintenance of a Th2-biased microenvironment. In clinical and translational research, human CCL18 is widely studied as a biomarker of alternative macrophage activation and fibrosis and is considered a potential therapeutic target in chronic inflammatory and fibrotic disorders.