L-selectin-mediated leukocyte recruitment. The initial capture of free-flowing leukocytes is mediated by L-selectin on leukocytes binding to endothelial ligands constitutively expressed on HEV, inflamed vessels, or on previously adhered leukocytes (secondary tethering). L-selectin alone mediates fast rolling, but synergistic interactions between L-selectin and integrin functions slow the rolling velocity of the leukocyte, facilitating activation by chemoattractants (e.g., chemokines) presented on the endothelium. Leukocyte activation (through L-selectin ligation and chemokines) induces high affinity integrin-mediated firm adhesion. Ultimately, the leukocyte transmigrates through the vascular endothelium and into the tissue. PAF, platelet-activating factor; CXCR4, CXC chemokine receptor 4.