1. A chemical peptide produced in the blood when tissues are injured that plays a role in inflammation.
2. A biologically active polypeptide, consisting of nine amino acids, that forms from a blood plasma globulin and mediates the inflammatory response, increases vasodilation, and causes contraction of smooth muscle.
3. A substance derived from plasma proteins which plays an important role in any of the reactions of the body, including inflammation.

Its primary action is in producing dilatation of the arteries and veins. It has also been described as "the most powerful pain-producing agent known".