Current concepts in lymphocyte homing and recirculation.

The immune system consists of a complex collection of leukocytes and dendritic cells that surveys most tissues in the body for the appearance of foreign antigens. For an efficient immune response, the interaction and co-localization of antigen-presenting cells, costimulatory helper cells and effector cells are crucial parameters. Therefore, the migration routes of antigen-presenting cells and potential antigen-specific lymphocytes merge in secondary lymphoid organs in order to increase the likelihood and speed of a lymphocyte finding its cognate antigen. Additionally, antigen-primed effector cells are directed to the tissue where they are most likely to encounter their cognate antigen. This highly organized and efficient antigen encounter is based on a continuous recirculation of antigen-specific lymphocytes between blood, peripheral tissue, and secondary lymphoid organs. Moreover, the efficacy of the immune system is further increased by the ability of different lymphocyte subsets to recirculate only through distinct tissues. The scope of this review is to outline the concept and mechanisms of lymphocyte homing and recirculation and to discuss the significance for the immune defense. Current models in leukocyte homing and recirculation and the underlying molecular functions of implicated cell adhesion molecules, chemokines, and chemokine receptors are discussed.