Schematic illustration of leukocyte anchor formation and stabilization mechanism in response to hydrodynamic force. Leukocytes attached to VCAM-1 via high affinity VLA-4 stretch microvilli in response to fluid flow. Tension generated through integrins along F-actin activates PI3K and Rap1. Rap1 and/or PI3K localizes Rac GEFs to sites of force generation. Rac-dependent F-actin polymerization stabilizes the actin cytoskeleton. Leukocyte structural anchors form stabilizing VLA-4–mediated arrest and adhesion.