Fibrous filters are used widely in transfusion medicine to remove leukocytes (white cells) from platelet-rich plasma (PRP), which is a suspension of leukocytes and platelets in plasma (water and proteins). The interaction of injected leukocytes and platelets with fibers in a filter controls the effectiveness of the filter to selectively retain leukocytes and to allow platelets to pass through. We present comprehensive experimental results at three different spatial scales-cellular, pore, and macroscale-to study PRP filtration in fibrous filters. We used atomic force microscopy (AFM) to measure and compare adhesive forces of the cells brought in contact with clean and protein-coated materials. We then extended cellular-scale experiments to pore-scale flow systems using transparent microfluidic platforms (filter chip) to visualize cell movement in interstitial space and cell capture to microposts under precisely controlled laminar and low-Stokes'-number conditions. We tested filter chips with different surface wettabilities; at the macroscale, we used commercial PRP filters. AFM and filter-chip experiments revealed that plasma proteins play important roles in modifying surface properties to adhere incoming cells. Filter-chip and commercial-filter experimental results showed that adhesion of both leukocytes and platelets decreases at higher shear rates. This reduction was more pronounced for platelets than for leukocytes, which means that results can be used to optimize the selective filtration process.