P-selectin (CD62P), a cell adhesion molecule expressed on the surface of activated vascular endothelial cells and platelets. In SCD, factors like inflammation and hemolysis trigger the expression of P-selectin. This molecule then binds to its ligand (e.g., PSGL-1) on circulating sickle red blood cells, leukocytes, and platelets, promoting their adhesion to the vessel wall and to each other. This multicellular adhesion is a critical initiating step in vaso-occlusion, which blocks blood flow and leads to the severe pain and organ damage characteristic of VOCs.