CCL17 was initially isolated from phytohemagglutinin-stimulated PBMCs. CCL17 is constitutively expressed in the thymus and, under activation, in several cell types. CCL17-mediated recruitment of Th2 cells and CLA+ CD4+ T cells plays a key role in allergic diseases like atopic dermatitis, allergic asthma, allergic rhinitis, and allergic contact dermatitis. In addition, CCL17 has been detected in idiopathic pulmonary fibrosis. Also, CCL17 and CCL22 secreted by dendritic cells seem to mediate the recruitment of regulatory T cells to sites of inflammation in patients with chronic hepatitis. CCL17 is upregulated by several combinations of cytokines: TNF-α, IL-4, and IFN-γ in bronchial epithelial cells; TGF-β1, TNF-α, and IFN-γ in human keratinocyte cell lines; and histamine and prostaglandin E2P in immature dendritic cells.????