In the 9 years since the last review on leukocyte and endothelial interactions was published in this journal many of the critical structures involved in leukocyte adherence to and migration across endothelium have been elucidated. With the advent of cell and molecular biology approaches, investigations have progressed from the early descriptions by intravital microscopy and histology, to functional and immunologic characterization of adhesion molecules, and now to the development of genetically deficient animals and the first phase I trial of "anti-adhesion" therapy in humans. The molecular cloning and definition of the adhesive functions of the leukocyte integrins, endothelial members of the Ig gene superfamily, and the selectins has already provided sufficient information to construct an operative paradigm of the molecular basis of leukocyte emigration. The regulation of these adhesion molecules by chemoattractants, cytokines, or chemokines, and the interrelationships of adhesion pathways need to be examined in vitro and, particularly, in vivo. Additional studies are required to dissect the contribution of the individual adhesion molecules to leukocyte emigration in various models of inflammation or immune reaction. Certainly, new adhesion structures will be identified, and the current paradigm of leukocyte emigration will be refined. The promise of new insights into the biology and pathology of the inflammatory and immune response, and the potential for new therapies for a wide variety of diseases assures that this will continue to be an exciting area of investigation.
