Figure 1. Proteinase and nonproteinase allergen-induced inflammation. Proteinase allergens cleave fibrinogen into fibrinogen cleavage products [a]. Nonproteinase allergens activate endogenous proteinases, such as thrombin [b], to cleave fibrinogen into cleavage products. Fibrinogen cleavage products act as ligands for toll-like receptor 4 (TLR4) expressed on cells including macrophages and airway epithelial cells [c]. Activated TLR4 stimulates recruitment of innate immune cells and airway inflammation [d].