Abstract: The invention relates to a method and system for extracorporeally treating the blood of a patient having diseased tissue or a condition, such as cancer, which results from or depends upon a dysfunction of the patient's immune system. In accordance with the invention, blood is withdrawn from the pateint, a low molecular weight fraction of the blood containing immunosuppressive components which retard or prevent immune system responses is selectively separated from the withdrawn blood, preferably by means of an ultrafilter, and treated blood is returned to the patient to initiate an immune response.