A fluid processing system, comprising: a blood separation chamber configured to separate mononuclear cells from a plasma constituent of blood; an outlet flow path associated with the blood separation chamber for alternately removing at least a portion of said plasma constituent and at least a portion of said mononuclear cells from the blood separation chamber, wherein the outlet flow path includes a valve station movable between first and second conditions to change the direction of flow of fluid through the outlet flow path; a mononuclear cell collection container in fluid communication with the outlet flow path; and a controller programmed to execute a mononuclear cell collection procedure including a build-up phase in which said at least a portion of said plasma constituent flows through the outlet flow path and the controller causes the valve station to be in the first condition to direct the flow of said at least a portion of said plasma constituent through the outlet flow path away from the mononuclear cell collection container while a volume of the mononuclear cells increases in the blood separation chamber, and a harvest phase in which said at least a portion of said mononuclear cells flows through the outlet flow path and the controller causes the valve station to be in the second condition to direct the flow of said at least a portion of said mononuclear cells through the outlet flow path to the mononuclear cell collection container, wherein the controller is further programmed to allow an operator to selectively transition from the build-up phase to the harvest phase and/or from the harvest phase to the build-up phase substantially instantaneously. a blood separation chamber configured to separate mononuclear cells from a plasma constituent of blood; an outlet flow path associated with the blood separation chamber for alternately removing at least a portion of said plasma constituent and at least a portion of said mononuclear cells from the blood separation chamber, wherein the outlet flow path includes a valve station movable between first and second conditions to change the direction of flow of fluid through the outlet flow path; a mononuclear cell collection container in fluid communication with the outlet flow path; and a controller programmed to execute a mononuclear cell collection procedure including a build-up phase in which said at least a portion of said plasma constituent flows through the outlet flow path and the controller causes the valve station to be in the first condition to direct the flow of said at least a portion of said plasma constituent through the outlet flow path away from the mononuclear cell collection container while a volume of the mononuclear cells increases in the blood separation chamber, and a harvest phase in which said at least a portion of said mononuclear cells flows through the outlet flow path and the controller causes the valve station to be in the second condition to direct the flow of said at least a portion of said mononuclear cells through the outlet flow path to the mononuclear cell collection container, wherein the controller is further programmed to allow an operator to selectively transition from the build-up phase to the harvest phase and/or from the harvest phase to the build-up phase substantially instantaneously. a build-up phase in which said at least a portion of said plasma constituent flows through the outlet flow path and the controller causes the valve station to be in the first condition to direct the flow of said at least a portion of said plasma constituent through the outlet flow path away from the mononuclear cell collection container while a volume of the mononuclear cells increases in the blood separation chamber, and a harvest phase in which said at least a portion of said mononuclear cells flows through the outlet flow path and the controller causes the valve station to be in the second condition to direct the flow of said at least a portion of said mononuclear cells through the outlet flow path to the mononuclear cell collection container, wherein the controller is further programmed to allow an operator to selectively transition from the build-up phase to the harvest phase and/or from the harvest phase to the build-up phase substantially instantaneously.