CCL17 is a powerful chemokine produced in the thymus and by antigen-presenting cells like dendritic cells, macrophages, and monocytes. CCL17 plays a complex role in cancer. It attracts T-regulatory cells allowing for some cancers to evade an immune response. However, in other cancers, such as melanoma, an increase in CCL17 is linked to an improved outcome. CCL17 has also been linked to autoimmune and allergic diseases. Classification CCL17 (CC chemokine ligand 17) was initially named TARC (thymus- and activation-regulated chemokine) when first isolated in 1996. It was later renamed CCL17 as the naming conventions for all cytokines were updated to standardize names. Function Cytokines, like CCL17, help cells communicate with one another, and stimulate cell movement. Chemokines are a type of cytokine that attract white blood cells to sites of inflammation or disease. CCL17 as well as its partner chemokine CCL22 induce chemotaxis in T-helper cells. They do this by binding to CCR4, a chemokine receptor expressed on type 2 helper T cells, cutaneous lymphocyte skin-localizing T cells, and regulatory T cells. CCR4 is also expressed by T cells involved in adult T-cell leukemia/lymphoma and cutaneous T cell lymphomas, making its ligands (namely CCL17) an attractive target for novel therapies as described below. CCL17 is one of the few chemokines that are not stored in the body, except in the thymus; these chemokines are made when needed by dendritic cells, macrophages, and monocytes. CCL17 is expressed constitutively in the thymus, but only transiently in phytohemagglutinin-stimulated peripheral blood mononuclear cells. CCL17 can also be detected in other tissues such as the colon, small intestine, and lung. Granulocyte-macrophage colony-stimulating factor (GM-CSF) upregulates CCL17 production in monocytes and macrophages. Dendritic cells will produce large quantities of CCL17 when stimulated with IL-4 or TSLP. CCL17 was the first CC chemokine identified that interacted with T cells with high affinity. CCL17 was also found to interact with monocytes, but with less affinity. It does not interact with granulocytes. It acts as a powerful chemoattractant to T-helper cells and T-regulatory cells because both can express CCR4. Cancer This chemokine is very important in the human body’s response to cancers. While it sometimes allows cancer to invade more rapidly, it more often helps the human body fight cancer. Some cancers that form tumors, such as breast cancer, produce CCL17 which draws T regulatory cells into the area, enhancing the cancer’s ability to invade. On the other hand,