The security system of the body against pathogenic invaders includes leukocytes that efficiently scan the organism. Leukocytes within the vasculature utilize the comprehensive circulatory system to examine the blood vessels of the entire body for signs of e.g. bacteria displayed on vascular endothelial cells. Upon infection, a successful immune response is dependent on prompt recruitment of leukocytes from the bloodstream to the afflicted site where they exert their effector functions. A critical aspect of leukocyte recruitment out of vasculature is the chemotactic gradient that guides leukocytes over the blood vessel wall, and further through the extracellular matrix towards the affected site. Leukocyte recruitment is a strictly regulated cascade of events involving different molecular mechanisms. To rapidly and efficiently reach their target, specific interactions between circulating leukocytes and vascular endothelium orchestrates leukocyte activation and guides them already within blood vessels to optimal transmigration sites at endothelial loci close to the source of inflammation.