Blood separation systems and methods separate mono nuclear cells from whole blood. The systems and methods introduce whole blood into a rotating chamber for separation into red blood cells, a plasma constituent, and an interface carrying mono nuclear cells between the red blood cells and the plasma constituent. The systems and methods convey whole blood into the chamber while removing red blood cells and the plasma constituent and while maintaining the interface at a set location within the chamber. At an appropriate processing time, the systems and methods move the interface from the set location for removal from the chamber through an outlet path. The outlet path includes a sensing element to locate mono nuclear cells in the outlet path outside the chamber.