Regulation of lymphocyte traffic by adhesion molecules

Lymphocytes are antigen specific cells whose effector function is acquired through complex differentiation pathways. This implies, firstly, antigen en counter and recognition at specific sites, and, subsequently, the transitio n from a naive to a memory/effector phenotype. Clonotypically expanded cell s must then be capable of recirculating to the tissue where their effector function is needed. To this aim, defined receptor-counter receptor pairs ar e expressed on lymphocytes versus endothelial cells. Extravasation is there fore a key-process in this scenario. Indeed, different lymphocyte subsets d isplay distinct recirculation patterns and capability to migrate into lymph oid and non-lymphoid tissues. As a general rule, naive lymphocytes preferen tially migrate into secondary lymphoid organs, where all the requirements f or effective antigen presentation and differentiation are available; in con trast, memory/effector lymphocytes preferentially migrate to peripheral tis sues, such as skin and mucosa. We review here the molecular events that reg ulate leukocyte extravasation and the specific migration properties acquire d by both naive and memory/effector lymphocytes under physiological and pat hological conditions.