Homing and migration: the endless journey of lymphocytes

The immune system consists of a complex collection of leukocytes and dendri tic cells that surveys most tissues in the body for appearance of foreign a ntigens. For an efficient immune response, the interaction and co-localizat ion of antigen-presenting cells, costimulatory helper cells and effector ce lls are crucial parameters. Therefore the migration routes of antigen prese nting cells and of lymphocytes merge in secondary lymphoid organs in order to increase the likelihood and speed of a lymphocyte, finding its cognate a ntigen. Additionally, antigen primed effector cells are directed to the tis sue where they are most likely to encounter their cognate antigen. This hig hly organized and efficient antigen encounter is based on continuous re-cir culation of lymphocytes between blood, peripheral tissues and secondary lym phoid organs. Moreover, the efficacy of the immune system is further increa sed by the ability of different lymphocyte subsets to recirculate through d istinct tissues. The scope of this review is to outline the concept and mec hanisms of lymphocyte homing by underlying the molecular function of implic ated cell adhesion molecules, chemokines and chemokine receptors, and to pr opose mechanisms by which homing physiology regulates the homeostasis of im munological resources.