[produced in response to specific stimuli by a variety of cell types, including neutrophils, basophils, platelets, and endothelial cells; several molecular species of platelet activating factor have been identified which vary in the length of the O-alkyl side chain; an important mediator of bronchoconstriction. ( CSP )] (UMLS (NCI) C0032172) =Lipid; Immunologic Factor =Blood Coagulation Factor;