Proposed mechanism underlying the pro-thrombotic action of C5a/C5aR1 signaling. Upon engagement with its receptor C5aR1, C5a induces the activation of RalA, which leads to the exocytosis of Weibel-Palade bodies (WPB). Then, vWF and P-selectin are exocytosed from WPBs, providing an adhesive scaffold on which circulating vWF is in turn recruited. As a consequence for the above events, massive platelet adhesion and aggregation occur on the endothelial cell surface.