Washing red blood cells means that after the red blood cells are separated from the plasma, they are washed 3-6 times with saline through a cell separator to remove most of the white blood cells and most of the plasma proteins. Most of the leukocytes and most of the plasma proteins are removed, and most of the cellular debris, metabolites, anticoagulants and various ions are also removed, and about 30% of the red blood cells are lost during the washing process. Since washing red blood cells washes away most of the white blood cells and various ions, it can significantly reduce the incidence of adverse reactions during blood transfusion, and is commonly used in patients with previous transfusion reactions and patients with transfusion allergies. At the same time, for patients with hyperkalemia and hepatic and renal insufficiency, it is also necessary to transfuse washed red blood cells. In addition, patients with autoimmune diseases, most commonly autoimmune hemolytic anemia, hemolytic disease of the newborn, and paroxysmal sleep hemoglobinuria, should also be treated with transfusion of washed red blood cells.