eicosanoid

An eicosanoid is a short-range chemical signal made from fat, on the spot, when a cell is disturbed. Unlike classic hormones that travel through the bloodstream to distant glands' targets, eicosanoids usually act on the very cell that made them or on near neighbors, then break down within seconds. They are best thought of as local messengers rather than long-distance hormones.

All eicosanoids are built from arachidonic acid, a twenty-carbon fatty acid pried out of the cell membrane by enzymes when a cell is injured or stimulated. From this raw material, different enzyme pathways produce prostaglandins, thromboxanes, and leukotrienes, each with distinct jobs in inflammation, pain, fever, blood clotting, and smooth-muscle contraction.

Because they are made and destroyed locally, eicosanoids blur the textbook line between a hormone and a paracrine signal. Their importance in medicine is enormous: aspirin and ibuprofen work by blocking the enzymes that make prostaglandins, which is how they relieve pain and fever.