Activated leukocytes and the hemostatic system

Activated leukocytes are capable of activating the blood-clotting system. Upon adequate stimulation (e.g., by endotoxin, by lectins, in the course of immune reactions, and in response to lymphokines of T cell origin), macrophages and monocytes synthesize tissue thromboplastin and expose it on their surface. Leukocyte proteases may interfere with blood coagulation by degrading clotting factors, in particular factors V, VIII, and XIII. Furthermore, these enzymes act as fibrinolytic agents, which also cleave fibrinogen. Leukocyte elastase attacks the platelet glycoproteins Ib (the receptor for von Willebrand factor) and V (a thrombin substrate) and at the same time exposes the platelet fibrinogen receptor. Platelet-activating factor of leukocyte origin may induce platelet aggregation and is a powerful potentiator of other inducers of platelet activity. The last-mentioned property has also been reported for leukotrienes. Activated leukocytes and their products play an important role in the pathogenesis of important disorders of the hemostatic system, with disseminated intravascular coagulation as the most prominent example.