Inertial microfluidics has recently drawn wide attention as an efficient, high-throughput microfluidic cell separation method. However, the achieved separation resolution and throughput, as well as the issues with cell dispersion due to cell-cell interaction, have appeared to be limiting factors in the application of the technique to real-world samples such as blood and other biological fluids. In this paper, we present a novel design of a spiral inertial microfluidic (trapezoidal cross-section) sorter with enhanced separation resolution and demonstrate its ability in separating/recovering polymorphonuclear leukocytes (PMNs) and mononuclear leukocytes (MNLs) from diluted human blood (1-2% hematocrit) with high efficiency (>80%). PMNs enriched by our method also showed negligible activation as compared to original input sample, while the conventional red blood cell (RBC) lysis method clearly induced artificial activation of the sensitive PMNs. Therefore, our proposed technique would be a promising alternative to enrich/separate sensitive blood cells for therapeutic or diagnostic applications.