Platelet activating factors

Platelet activating factors is a cell membrane derived lipid (Like eicosanoids) with intense biological activity. Platelet activating factor is a misleading name since it has its effects on a variety of other cells and is an important mediator in allergic, asthmatic and inflammatory responses.

It is produced by neutrophils, macrophages, eosinophils, mast cells, basophils and platelets in response to various stimuli. It acts on specific G protein coupled receptors that are found in many cell types.

Antigen-antibody reaction and a wide variety of mediators stimulate PAF synthesis in a Ca⁺⁺ dependent manner on demand. It is degraded and the acyl glycerophosphate formed gets incorporated in the membrane.

Pharmacological Actions of Plate activating factors

1. On platelets– Causes aggregation and release TXA₂.
2. On WBC– Chemotactic to neutrophils, eosinophils and monocytes. Stimulates neutrophils to aggregate  and stick to vascular endothelium and migrate across it to the site of infection. It also promotes release of lysosomal enzymes and LTs.
3. On blood vessels– Vasodilatation probably by the release of EDRF.
4. On visceral smooth muscle– causes contraction of intestinal and uterine smooth muscles and bronchoconstriction. 
5. On stomach– It is ulcerogenic and causes erosions and mucosal bleeding.

Pathophysiological role of Plate activating factors

PAF plays a role in inflammation, bronchial asthma, anaphylactic shock, hemostasis and thrombosis.