. Exocytosis of P-selectin by ECs. P-selectin is preformed and stored in Weibel Palade bodies (WPBs). It is found to be solely present or co-stored in WPBs with von Willebrand Factor (vWF) or angiopoietins (Ang). Upon extracellular stimulation, WPBs exocytose to the cell surface via the activation of Ral-GTP from Ral-GDP. WPB-containing vWF is also driven and translocated to the plasma membrane by SNARE. The rapid surface expression of P-selectin mediates the initial recruitment of leukocytes to ECs by rolling and tethering, which is important during the early development of allergic inflammation.