Chemokines are a family of small chemotactic cytokines, or proteins secreted by cells. Chemokines share the same structure similarities such as small size, and the presence of four cysteine residues in conserved locations in order to form their 3-dimensional shape. Some of the chemokines are considered pro-inflammatory which can be induced to recruit cells of the immune system to a site of infection during an immune response, while others are considered homeostatic and are implied in controlling the migration of cells during normal processes of tissue maintenance and development. There are four members of the chemokine family: C-C kemokines, C kemokines, CXC kemokines and CX3C kemokines. The C-C kemokines have two cysteines nearby the amino terminus. There have been at least 27 distinct members of this subgroup reported for mammals, called C-C chemokine ligands-1 to 28. Chemokine ligand 21(CCL21), also known as 6Ckine, exodus-2, and secondary lymphoid-tissue chemokine(SLC), is a small cytokine belonging to the C-C chemokine family. CCL21 takes its name 6Ckine for its consititutively six conserved cysteine residues but not four cysteines typical to chemokines. CCL21 has function in ininducing vigorous calcium migrations and chemotactic responses.
