Systems and methods for collecting diluted mononuclear 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 platelets and mononuclear cells between the packed red blood cells and the plasma constituent,
• a first collection container,
• a second collection container,
• a pump,
• a controller operable to convey whole blood into the inlet region while removing packed red blood cells and the plasma constituent from the chamber, the controller including an interface control unit operative (i) in a first condition to retain platelets and mononuclear cells in the chamber to enable removal of platelet-poor plasma from the chamber in a path that leads to the first container and not the second container;
• (ii) in a second condition to retain mononuclear cells in the chamber while enabling removal of platelet-rich plasma from the chamber in a path that bypasses the first and second containers;
• and (iii) in a third condition to enable removal of mononuclear cells from the chamber in a path that leads to the second container and not the first container, and
• the controller further operating the pump to direct platelet-poor plasma from the first container to the second container to dilute the removed mononuclear cells in the second container.