A blood separation system operates in a first mode to convey whole blood into the inlet region (48) of a blood processing chamber (38) for centrifugal separation into packed red blood cells, a plasma constituent, and an interface, which carries mononuclear cells, between the packed red blood cells and the plasma constituent. The system removes packed red blood cells and the plasma constituent from the chamber (38), while maintaining the interface within the chamber (38). The system operates in a second mode to remove the interface from the chamber (38) by conveying packed red blood cells into the inlet region (48). An outlet path (46) conveys the removed interface from the chamber (38). The outlet path (46) includes a first sensing element (OS) to locate mononuclear cells in the removed interface and provide a sensed output upon locating mononuclear cells.