CC chemokine 21 (CCL21), also known as secondary lymphoidorgan chemokine (SLC), is a ligand for the chemokine receptor CCR7. CCL19 and CCL21 are the only ligands for CCR7, which is expressed by various subsets of immune cells. CCL19 and CCL21 function to recruit CCR7 expressing antigen presenting cells, like dendritic cells and naive T-cells, to the lymph nodes. CCL21 seems to be the critical CCR7 ligand regulating dendritic cell homeostasis and function. CCL21 has an unusual extended C-terminal tail for a chemokine. Only one gene exists for this chemokine in human beings, whereas mice have two. CCL21 Inhibit hemopoiesis and stimulates chemotaxis. Chemotactic in vitro for thymocytes and activated T-cells, but not for B-cells, macrophages, or neutrophils. Shows preferential activity towards naive T-cells.