System and methods for harvesting mononuclear cells by recirculation of packed red blood cells

A blood separation system comprising

• a chamber for rotation about a rotational axis, the chamber including an inlet region where whole blood enters for separation into packed red blood cells, a plasma constituent, and an interface carrying mononuclear cells between the packed red blood cells and the plasma constituent,
• a whole blood inlet path communicating with the inlet region to convey whole blood into the inlet region,
• a packed red blood cell outlet path to convey packed red blood cells from the chamber,
• a reservoir outside the chamber to collect packed red blood cells,
• a controller operable in a first mode to convey whole blood through the whole blood inlet path into the inlet region while removing packed red blood cells and the plasma constituent from the chamber and while maintaining the interface within the chamber, the controller also being operable in a second mode to remove the interface from the chamber by conveying packed red blood cells from the reservoir into the inlet region through the whole blood inlet path and without flow of packed red blood cells through the packed red blood cell outlet path, and
• an outlet path to convey the removed interface from the chamber, including a first sensing element to locate mononuclear cells in the removed interface and provide a first sensed output upon locating mononuclear cells.