Dimerized P-selectin, which is expressed on the membrane of activated platelets and the endothelium, binds PSGL-1 on leukocytes to mediate adhesion and signaling, leading to inflammation. These pathways lead to proteolytic shedding of P-selectin, releasing sP-selectin, a monomer. Circulating monomeric sP-selectin binds PSGL-1 less well and does not induce leukocyte signaling, leading to inflammation.