It is well known that when a suspension of cells flows in small vessels (arterioles or venules), there exists a cell-free layer of a few microns adjacent to the vascular walls. Using an in vitro model, we show experimentally that for a fixed flow rate a geometrical constriction in the flow can artificially enhance the cell-free layer. Also, we show that rapid variation of the geometry coupled to the deformability of the cells can dramatically modify their spatial distribution in the channel. The effects of the constriction geometry, flow rate, suspending fluid viscosity, cell concentration, and cell deformability are studied and the results are interpreted in terms of a model of the hydrodynamic drift of an ellipsoidal cell in a shear flow. We propose a microfluidic application of this focusing effect for separation of the red blood cells from the suspending plasma.