One-step purification of white blood cells (WBCs) is essential to automate blood sample preparation steps for WBC analysis, but conventional methods such as red blood cell (RBC) lysis and density-gradient centrifugation typically require harsh chemical or physical treatment, followed by repeated manual washing steps. Alternative microfluidic separation methods show limited separation performances due to the trade-off between purity and throughput. Herein, an integrated microfluidic device is developed to decouple the trade-off by synergistically combining a slant array ridge-based WBC enrichment unit as a throughput enhancer and a slant, asymmetric lattice-based WBC washing unit as a purity enhancer. The enrichment unit can maintain a high sample-infusion throughput while lowering the flow rate into the washing unit, thus enabling WBC-selective washing without significant influence by the overwhelming number of RBCs and inertial forces. The device delivers efficient separation performances by rejecting 99.9% of RBCs as well as 99.9% of blood plasma from canine and human whole blood in a single round of purification at a high throughput of 60 μL/min. The purified WBC population well preserves the composition of lymphocyte subpopulations, the major components of the adaptive immune system, thus providing the potential for the integrated device to be used as an essential sample-preparation tool for immunologic investigations.