We investigate the shear thinning of red blood cell-dextran suspensions. Microscopic images show that at low polymer concentration, aggregation increases with increasing concentration until it reaches a maximum and then decreases again to non-aggregation. This bell-shape dependency is also deduced from the rheological measurements, if the data are correctly normalized by the viscosity of the suspending phase since a significant amount of polymers adsorb to the cell surfaces. We find that the position of the maximum of this shear rate-dependent bell shape increases with increasing viscosity of the suspending phase, which indicates that the dynamic process of aggregation and disaggregation is coupled via hydrodynamic interactions. This hydrodynamic coupling can be suppressed by characterizing a suspension of 80 % hematocrit which yields good agreement with the results from the microscopical images.