Many diseases are related to specific disease-causing substances that are abnormally produced or present in the body, especially in the blood, such as anti-DNA antibodies that can cause systemic lupus erythematosus, anti-glomerular basement membrane antibodies that can cause acute glomerulonephritis, immunoglobulin M that can cause hyperviscosity syndrome, and poisoning from certain drugs or poisons. If these disease-causing substances can be removed directly or quickly, the disease can often be controlled quickly in a short period of time. Plasma exchange is one such method. We know that blood is composed of blood cells and plasma, and plasma contains a large number of various components, including various immunoglobulins, lipoproteins, coagulation factors, albumin, small molecule peptides and various ions, etc., with different specific gravity, density and molecular weight. If we use membrane filtration or other methods to separate blood cells and plasma, then discard the plasma containing disease-causing substances and return the blood cells to the body, while replenishing normal human plasma or plasma substitutes, we can non-specifically remove certain disease-causing substances from the plasma and achieve the purpose of treating diseases, which is called plasma replacement therapy. With the continuous development of plasma replacement technology, it is now possible to further divide plasma into many subcomponents and selectively remove them according to the therapeutic needs. Membrane filtration involves drawing the patient’s blood out of the body and passing it through a special plasma separation membrane, which controls the size of the pores on the membrane so that only plasma components, such as various proteins and solutes of lower molecular weight than proteins, are allowed to pass through, while all blood cell components are retained. It should be noted that although plasma exchange has its own special therapeutic effect, it is not a fundamental therapeutic measure after all, and must be applied clinically in conjunction with other necessary therapeutic measures such as drug therapy.