Role of contact activation in thrombosis protection in Klkb1^−/−. (A,D) Survival rate in WT, Klkb1^−/−, and F12^−/− mice after challenge with collagen/epinephrine (Col/Epi) (A) or long-chain bacterial polyphosphate (LC polyP) (D). The ratios on top of each column represent the number of mice surviving 30 minutes. (B,E) Hematoxylin and eosin (H&E) staining of lung sections from Col/Epi- (B) or LC polyp- (E) challenged Klkb1^+/+ and Klkb1^−/− mice. The red arrows point to representative occluded vessels. Images were obtained from a Leica SCN 400 Slide Scanner equipped with a Hamamatsu line-sensor color camera and a ×40/0.65 objective lens. The images were made through a ×2 tube lens at ×40 final magnification. (C) (Col/Epi) and (F) (LC polyP). Number of vessel occlusions quantified from H&E staining. Vessel occlusions per visual field were counted at ×40. Mean ± standard error of the mean (SEM) of 100 fields per group are shown. *Significant difference (P < .05) among the groups on 1-way analysis of variance (ANOVA).