Leukocytes are white blood cells that help in fighting infection (at site of injury, infection or inflammation). Cytokines are released which stimulate endothelial cells, which lines adjacent blood vessel. Endothelial cells express surface proteins called Selectins.
Selectins binds to carbohydrates in membrane of leukocytes causing it to stick to walls of blood vessel. Binding interaction is insufficient lower affinity that leukocytes can roll on along the vessel wall in search for points to exit.
Leukocytes adhere tightly and squeeze between endothelial cells without disturbing the vessel walls and crawl out of the blood vessel to adjacent connective tissue