Molecular control of leukocyte trafficking internal regulatory circuits of the immune system: Leukocyte circulation and homing

Leukocyte migration is essential for their ability to react to inflammatory stimuli at various locations within the body. However, leukocyte movement is also crucial during non-inflammatory processes such as haematopoietic development and routine passage through secoandary lymphoid organs, which is the key to effective antigen presentation. The importance of cell migration in the establishment and maintenance of an intact immune system has been demonstrated by numerous observations of immune defects in chemokine receptor deficient mice. This review describes the role of chemokines, their receptors and adhesion molecules in the regulation of the immune response during inflammatory and homeostatic conditions. In particular, we describe the molecular events regulating the recruitment of innate cells in response to infection and inflammation, leukocyte trafficking during development processes such as haematopoiesis and thymic naturation of T cells, and regulatory circuits that ensure immune surveillance and communication between innade and adaptive components of immunity.