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Emerging aspects of leukocyte migration

The motility of its cellular components is a key feature of the immune system since it allows the selective recruitment of cells to defined places and niches not only to combat invading pathogens but also to pass through tightly controlled programs of immune cell development and maturation. In nature, two profoundly different processes contribute to immune cell motility: passive transport in blood and lymph fluid between different lymphoid and nonlymphoid organs, and active migratory processes that control entry and exit to and from lymphoid organs as well as their precise positioning therein. Furthermore, active cell migration is a key process that allows the entry and exit of effector cells to places of infection and inflammation. It is generally accepted that the passive, bloodmediated transport of immune cells to lymphoid and nonlymphoid organs occurs as a random, noncontrolled process. In contrast, a plethora of studies in the past two decades have revealed many of the cellular and molecular cues that control immune cell migration to and within defined compartments. While passive transport allows rapid cellular distribution and dissemination throughout the body, active migratory processes are of paramount importance to tightly control the immune system. Passive transport and active migration provide the basis of all immune responses not only in vertebrates but also in all multicellular organisms that harbor motile immune cells with defined effector functions. This Viewpoints series addresses several emerging aspects of leukocyte migration. Key players in immune cell migration