CytokineCytokines are small protein molecules that are the core of communication between immune system cells, and even between immune system cells and cells belonging to other tissue types. They are actively secreted by immune cells. Their action is often local, but sometimes can have global effects on the whole body.
Cytokines act by binding to their cell-specific cytokine receptor. This receptor is located in the cell membrane and allows a signal cascade to start in the cell, that eventually will lead to biochemical and phenotypical changes in the target cell.
See also: Chemokines, Interleukines, Interferon