Extracorporeal blood treatment is performed daily on a large number of patients by use of for example a dialysis filter, plasma filter or a centrifuge. The purpose of the treatment is to separate minor components and molecules in a liquid or in the blood from larger ones, for example in connection with different disease conditions or with a view to extracting blood plasma, target substances such as blood components or molecules from for example blood donors. According to the present invention two separation steps are used together with a solution or a suspension containing at least one component, which specifically may bind to the component or the blood component to be specifically reduced, or to be refined/extracted during the treatment or during the blood treatment.