Activated leukocytes expressing adhesion molecules and integrins roll and attach to the vascular endothelium. Successful diapedesis requires appropriate ligation of adhesion molecules, selectins, and integrins, signaling to both the infiltrating leukocyte and the brain endothelium. Expression of CXCL12 on the basolateral surface of endothelial cells recruits CXCR4⁺ T cells. However, retention of cells in the perivascular space occurs in the presence of high concentrations of CXCL10. Continued migration puts cells in contact with the glia limitans, which is composed of a highly structured wall of astrocytes. Further positive migratory signals, including chemokines, from these and surrounding cells may allow leukocyte migration into the parenchyma.