The next step in the process of leukocyte recruitment is migration of the leukocytes through the endothelium, called transmigration or diapedesis. Transmigration of leukocytes occurs mainly in post-capillary venules. Chemokines act on the adherent leukocytes and stimulate the cells to migrate through interendothelial spaces toward the chemical concentration gradient, that is, toward the site of injury or infection where the chemokines are being produced. REF: ROBBINS pathology 10th edition