Chemokines are tiny protein molecules that form a subfamily of the cell signalling molecules or cytokines. These tiny proteins are secreted by cells to induce chemotaxis in nearby cells. Chemotaxis refers to when cells direct their movement according to the presence of chemicals in their surrounding environment.
For example, the presence of a microbe or a foreign body triggers the release of chemicals that then direct immune cells to migrate towards the site of infection. Neutrophils are induced to leave blood vessels and migrate towards the site of infection where the invading body is present. Monocytes and immature dendritic cells are then recruited at a later stage. Chemokines are therefore chemotactic cytokines.
Chemokines are small in size with a molecular weight of around 8 to 10 kilodaltons (kDa). The chemokines also share structural similarity and usually contain four cysteine residues, which gives them their three dimensional shape.
Historically, the chemokines have been known by various different names including the SIS family, SIG family or SCY family of cytokines, the intercrines and the platelet factor-4 superfamily. Chemokines are present in all vertebrate animals as well as in some microbes including bacteria and viruses, but none have yet been identified in other non-vertebrates.
The role of certain chemokines is considered pro-inflammatory, with the proteins being recruited to an infection site during an immune response, while other chemokines are thought to have a homeostatic role, controlling cell migration as part of normal tissue growth and maintenance.
The action of a chemokine is mediated when it interacts with a chemokine receptor which is a member of the G protein-coupled receptor (GPCR) family. These are transmembrane receptors that are coupled to intracellular G-protein, which stimulates signal transduction pathways inside the cell when it is activated.
Nineteen types of chemokine receptor have so far been identified in mammals and they are found predominantly on the surface of white blood cells or leukocytes.