Systems and methods rotate a chamber (14), in which whole blood is centrifugally separated into an interface between the packed red blood cells and the plasma constituent. The interface carries platelets and mononuclear cells. The systems and methods include an interface control unit which is operative in a first state to enable removal of platelet-poor plasma (PPP) in a first container for use as a diluting liquid, and in a second state, to retain mononuclear cells (MNC) in the chamber (14), while removing platelet-rich plasma (PRP) from the chamber (14), bypassing the platelet-poor collection container, thereby maintaining its platelet-poor character, and in a third state to enable the removal of mononuclear cells (MNC) from the chamber (14) in a path (T13) that leads to a second container, where the mononuclear cells are collected.