Binds to atypical chemokine receptor ACKR4 and mediates the recruitment of beta-arrestin (ARRB1/2) to ACKR4 (PubMed:23341447). Binds to chemokine receptor CCR7 (PubMed:9507024). Does not bind to chemokine receptor CXCR3 (PubMed:10201891). Binds to G protein-coupled receptor GPR174 (By similarity). Inhibits 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. May play a role in mediating homing of lymphocytes to secondary lymphoid organs. {ECO:0000250|UniProtKB:P84444, ECO:0000269|PubMed:10201891, ECO:0000269|PubMed:23341447, ECO:0000269|PubMed:9507024}. This is the function of Ensembl gene identifier ENSG00000137077 (CCL21, C-C motif chemokine ligand 21).