Systems and methods are provided for reducing the number of leukocytes in cellular products like platelets collected in an on-line or continuous blood separation processes. The systems include first separation element (12) for separating blood into a suspension of platelets, second separation element (14) for reducing the number of leukocytes in a suspension of platelets, third separation element (16) for separating a suspension of platelets into a platelet-rich concentrate and a control means for operating separation elements (12, 14, 16) in various modes. The methods for obtaining a high quality blood product suited for therapeutic use are disclosed.