Human CCL21 (C-C motif chemokine ligand 21, also known as secondary lymphoid-tissue chemokine, SLC, or 6Ckine) is a homeostatic chemokine that plays a critical role in lymphocyte trafficking and organization of adaptive immune responses. CCL21 primarily signals through the CCR7 receptor, directing the migration of naïve T cells, central memory T cells, and mature dendritic cells to secondary lymphoid organs such as lymph nodes, spleen, and Peyer’s patches. In humans, CCL21 is produced mainly by stromal cells and high endothelial venules within lymphoid tissues, where it establishes chemokine gradients that guide immune cells into T-cell zones and facilitate interactions between antigen-presenting dendritic cells and lymphocytes required for activation of adaptive immunity. CCL21-mediated CCR7 signaling is essential for immune surveillance, lymph node organization, and efficient antigen presentation during infection. Dysregulation of CCL21 expression has been associated with chronic inflammatory diseases, autoimmune disorders, and cancer, where altered chemokine gradients can influence immune cell infiltration and tumor microenvironment dynamics. Because of its central role in lymphoid tissue organization and immune cell trafficking, human CCL21 is widely studied in vaccine responses, tumor immunology, and immune-mediated disease, and is being explored as a potential therapeutic target for enhancing immune activation or modulating inflammation.