Human CCL17 (C-C motif chemokine ligand 17), also known as thymus and activation-regulated chemokine (TARC), is a member of the CC chemokine family that plays a central role in regulating Th2-associated immune responses. CCL17 is produced primarily by dendritic cells, macrophages, endothelial cells, keratinocytes, and epithelial cells in response to inflammatory and type 2–associated cytokines. It functions predominantly through the CCR4 receptor to promote chemotaxis and activation of Th2 lymphocytes and regulatory T cells, contributing to allergic inflammation and modulation of adaptive immunity. In healthy individuals, circulating CCL17 levels are typically low, but concentrations increase significantly in allergic and inflammatory conditions. Elevated CCL17 has been strongly associated with atopic dermatitis, asthma, allergic rhinitis, eosinophilic disorders, and certain lymphomas such as Hodgkin lymphoma and cutaneous T-cell lymphoma, where it may contribute to Th2-skewed immune environments. In clinical and translational research, human CCL17 serves as a widely used biomarker of Th2 polarization and disease activity, particularly in atopic dermatitis, and is an important target for therapeutic strategies aimed at modulating type 2–driven inflammation.