Inflammation is a powerful system of host defense. The inflammatory mediators are the chemical substances which cause or participate in inflammation; they have short half lives, are degraded after their release, and produced in quick bursts, only as long as stimulus persists. Important for balanced and controlled mediator activity, the activity is rapid, specific, but short. Mediators originate either from plasma or cells. Plasma-derived mediators are present in plasma in precursor forms and must be activated to acquire their biologic activity, usually by a series of proteolytic cleavages. Cell-derived mediators are normally within intracellular granules and must be secreted or even synthesized de novo - due to response to specific stimulus. The major cellular sources are platelets, neutrophils, macrophages, and mast cells, but mesenchymal cells (endothelium, fibroblasts) and most epithelial cells can also be induced to produce some of the mediators.