Acute lymphocytic leukemia (ALL) is the most prevalent pediatric cancer, and the peripheral blood lymphoblast percentage is an important index for ALL diagnosis and prognosis. We describe a microfluidic device that isolates and enumerates peripheral blood lymphoblasts using affinity separations. The innovative use of a nonspecific ligand allows a widespread "net" for cancer cells, without a priori knowledge of the cancer type. Using lymphoblasts spiked into blood, we simulated leukemia cases with lymphoblast concentrations ranging from 1 to 30% of total leukocytes. Lymphoblasts were isolated using monoclonal antibodies for the Human Transferring Receptor (CD71). Anti-CD71 antibodies were found to be more effective for capturing lymphoblasts than commonly used, ALL-specific antibodies for CD7 and CD10. CCRF-CEM lymphoblasts were isolated in the chip with 82-97% purity, with lower concentrations tested (7%) still showing >80% purity for cell capture. Patient-derived ALL cell lines COG-LL-332 and COG-LL-317 were isolated in the chip with 80%-97% and 57% -92% of purity, respectively, with the initial spike concentrations as low as 1%. The ability to capture ALL lymphoblasts present in blood at low concentrations provides a novel approach for characterization of ALL cells, including patients with low leukemic burdens during and after therapy.