Rheological Mechanisms Contributing to WBC-Margination

Since the function of WBC is mainly fullfilled in the extra-vascular tissue, these cells have to emigrate from the circulation which is only used as a transport system. Before active emigration can take place the leukocytes must be in a position where they can interact with the endothelium through which they penetrate. This interaction can only be established if the WBC travel close to the vessel wall. Therefore the phenomenon of margination must be considered as a first step in the series of events constituting leukocyte extravasation. Since active locomotion of WBC within the flowing blood is impossible, leukocytes must be passively displaced towards the endothelial surface. This displacement can be favored or inhibited by the hydrodynamic and rheological flow conditions (1). Even if the mechanism of WBC-margination and the following steps leading to emigration are not yet fully understood, it is well known that these phenomena occur mainly in the small venules of the microcirculation (2, 8).