A device and/or methodology are described that include a mechanism for separating erythrocytes from other constituents of blood and for purifying leukocytes from blood. The separation and purification aspects may be provided in separate components or within the same component. The separation aspect assists in separating erythrocytes (red blood cells) from other cells in blood, such as by aggregation of the red blood cells. A suitable aggregation device or device component uses chambers with at least one small dimension (e.g., a microfluidic chip) to control the interaction of the blood with a solution containing a high molecular weight polymer (e.g., dextran) to achieve separation.