Interaction between coagulation and allergic inflammation in asthma. Coagulation is activated in the airways of patients with asthma by leak of clotting factors and TF expressed on various cell types, including alveolar epithelium, macrophages, and eosinophils. Fibrin deposition is further facilitated by decreased activity of the anticoagulant protein C system and inhibition of fibrinolysis by enhanced production of PAI-1. Allergens are responsible for an inflammatory response in the lungs, which is aggravated by proinflammatory effects of platelets and decreased cytoprotective effects of the PC system. PARs play an important role as the molecular link between coagulation and inflammation; these receptors are activated by proteases expressed by either allergens or factors involved the regulation of coagulation.