In this work, we investigate the effects of dielectrophoresis (DEP) on microfluidic immunocapture of prostate cancer and pancreatic cancer cells. We make novel measurements of these cancer cells' DEP response, and characterize the combination of DEP and immunocapture techniques as a function of shear stress in a Hele-Shaw flow cell with interdigitated electrodes. At the same applied electric field frequency, we demonstrate enhanced capture of cancer cells by attracting them to immunocapture surfaces with positive DEP and reduced nonspecific adhesion of peripheral blood mononuclear cells (PBMCs) by repelling them from immunocapture surfaces with negative DEP. Using an exponential capture model, we show that immunocapture performance is dependent on the applied DEP force sign and magnitude, cell and immunocapture surface chemistry, and shear stress experienced by cells flowing in the capture device. These data inform the simulation of cancer cell and blood cell capture probabilities to design future hybrid DEP and immunocapture device geometries with improved rare cell capture performance.