A phospholipid mediator of inflammation produced by a variety of cell types. Originally named for its ability to activate and aggregate platelets as part of the blood-clotting mechanism, it is now known to have a broad spectrum of biological effects. PAF is one of the front-line agents produced by activated macrophages in the initial phase of an innate immune response to infection. It contributes to the increased permeability of local blood vessels, helping to attract immune cells to the infection site, and activates not only platelets but also leucocytes, notably eosinophils, basophils, and neutrophils, thereby amplifying the inflammatory response. Its release from mast cells during an allergic reaction contributes to constriction of the airways in asthma and also to the reduction in blood pressure during anaphylaxis.