The microfluidic ratchet mechanism employed in this study overcomes some key limitations of both conventional microfiltration and microfluidic biophysical cell sorting systems to enable leukocyte separation from whole blood with 100% purity, as well as phenotypic sorting of leukocyte subpopulations. Microfiltration methods developed previously using pillars23, 24, weir23, 25, 26 and membrane pores23, 27 have all shown a tendency to clog, which increases hydrodynamic resistance, reduces selectivity and necessitates increased filtration pressure or periodic washing of the filter region. While secondary tangential flow partly alleviates device clogging23, 28–31, the oscillating flow used in microfluidic ratchet sorting enables clog-free and nearly perpetual processing of high-density whole blood samples. Since the deformation of cells is less than that which leukocytes normally experience when traversing through micro-capillaries in the body39, 40, this process does not adversely affect the viability of the leukocytes.