The trafficking of lymphocytes and other leukocytes plays a major role in the physiology of the itnmune system and the implementation of immune mechanisms. The recirculation of lymphocytes allows the full repertoire of antigenic specificities to be continuously represented throughout the body, and may also facilitate interactions between lymphocyte subsets and accessory cells required for antigen-specific effector functions. In addition, lymphocyte trafficking plays a role in segregating particular effector and memory subpopulations permitting the development of unique responses in, for example, mucosal versus non-mucosal sites. The selective extravasation of other classes of leukocytes, such as monocytes and neutrophils, contributes to inflammatory reactions resulting in the destruction of foreign microorganisms, clearance of tumors and the expulsion of parasites.

The first step reqtiired for lymphocyte entry into peripheral lymph nodes and mucosal lymphoid organs (Peyer's patches and appendix) from the blood is the recognition and adhesion to postcapillary venules lined by high endothelium (HEV). Lymphocytes as well as HEV employ specific cell surface molecules for this function and these or related molecules are also important in directing the extravasation of other leukocytes. In this manuscript we will briefly outline our view of the role of lymphocyte-HEV interactions in lymphocyte traffic, and will summarize our studies of some of the molecules involved. We will discuss the