Bovine CCL21 (C-C motif chemokine ligand 21, also known as secondary lymphoid-tissue chemokine, SLC, or 6Ckine) is a homeostatic chemokine that plays an essential role in lymphocyte trafficking and organization of immune responses in cattle (Bos taurus). CCL21 primarily signals through the CCR7 receptor, promoting chemotaxis of naïve T cells, central memory T cells, and mature dendritic cells to secondary lymphoid tissues such as lymph nodes and spleen. In bovine immune systems, CCL21 is produced mainly by stromal cells and endothelial cells within lymphoid tissues, where it guides immune cells into T-cell zones and supports interactions between antigen-presenting cells and lymphocytes necessary for initiation of adaptive immune responses. In bovine health, CCL21-mediated CCR7 signaling is important for effective immune surveillance and the development of protective responses against pathogens involved in diseases such as bovine respiratory disease complex (BRDC), mastitis, and mycobacterial infections including Mycobacterium bovis. Proper regulation of CCL21 contributes to lymph node architecture and efficient antigen presentation, whereas dysregulated chemokine signaling may influence chronic inflammation or impaired immune responses. As a key regulator of immune cell migration and lymphoid tissue organization, bovine CCL21 is an important molecule in studies of host–pathogen interactions, vaccine-induced immunity, and lymphoid immune function in cattle.