Chemotactic factor for T-lymphocytes but not monocytes or granulocytes. May play a role in T-cell development in thymus and in trafficking and activation of mature T-cells. Binds to CCR4. Expressed at high levels in thymus and at low levels in the lung, colon and small intestine. By phytohemagglutinin (PHA) in the peripheral blood mononuclear cells and by cytokines in monocytes. Belongs to the intercrine beta (chemokine CC) family. Was originally thought to be a ligand for CCR8. CCL17 entry