Red cell aggregation as a factor influencing margination and adhesion of leukocytes and platelets

Leukocytes and platelets must adhere to the wall of blood vessels to carry out their protective functions. Rheological factors influencing these processes are the delivery of the cells to the wall, referred to as margination, and the local shear rates and stresses at the wall. Margination requires leukocytes and platelets to be excluded from the central flow of the much more numerous red blood cells. This exclusion may be influenced by red cell aggregation. Red cell aggregation also influences development of plug flow in small vessels, which in turn modifies the wall shear rate and stress from those expected in ideal Poiseuille flow. Promotion of aggregation by added agents such as high molecular weight dextrans or by reduction in shear rate, increases margination of leukocytes and efficiency of attachment to the vessel wall. Interestingly, however, fewer studies exist for platelets, and these suggest that margination is actually promoted by increasing shear rate. Direct studies of the effects of red cell aggregation on platelets are required, but it appears that aggregation has different effects on delivery of platelets compared to leukocytes. These differences may represent adaptations for efficient adhesion of leukocytes and platelets in different regions of the circulation.