To minimize the chance of an adverse reaction during a transfusion, health care practitioners take several precautions. Before starting the transfusion, usually a few hours or even a few days beforehand, a technician mixes a drop of the donor's blood with the recipient's to make sure they are compatible. This procedure is called cross-matching.After double-checking labels on the bags of blood that are about to be given to ensure the units are intended for that recipient, the health care practitioner gives the blood to the recipient slowly, generally over 1 to 2 hours for each unit of blood. Because most adverse reactions occur during the first 15 minutes of the transfusion, the recipient is closely observed at first. After that, a nurse checks on the recipient periodically and must stop the transfusion if an adverse reaction occurs.Most transfusions are safe and successful; however, mild reactions occur occasionally, and severe and even fatal reactions, rarely. The most common reactions are fever and allergic reactions (hypersensitivity), which occur in about 1 to 2% of transfusions. Symptoms of an allergic reaction include itching, a widespread rash, swelling, dizziness, and headache. Less common symptoms are breathing difficulties, wheezing, and muscle spasms. Rarely, an allergic reaction is severe enough to cause low blood pressure and shock. Another rare reaction, called transfusion-related acute lung injury, or TRALI, is caused by antibodies in the donor's plasma. This reaction, which is more common when the donor is a woman who has been pregnant, may cause serious breathing difficulties. More general use of male donor plasma has decreased the number of people who have this reaction.Treatments are available that allow transfusions to be given to people who previously had allergic reactions to them. People who have allergic reactions to donated blood may have to be given washed red blood cells. Washing the red blood cells removes components of the donor blood that may cause allergic reactions. More commonly, the transfused blood is filtered to reduce the number of white blood cells (a process called leukocyte reduction). Leukocyte reduction is usually done by placing a special filter in the tubing through which the transfusion is flowing. Alternatively, the blood may be filtered before it is stored.Despite careful typing and cross-matching of blood, mismatches due to subtle differences between donor and recipient blood (and, very rarely, errors) can still occur that cause the transfused red blood cells to be destroyed shortly after the transfusion (a hemolytic reaction). Usually, this reaction starts as a general discomfort or anxiety during or immediately after the transfusion. Sometimes breathing difficulty, chest pressure, flushing, and severe back pain develop. Very rarely, the reactions become more severe and even fatal. A doctor can confirm that a hemolytic reaction is destroying red blood cells by checking to see whether hemoglobin released from these cells is in the person's blood and urine.Transfusion recipients can become overloaded with fluid. Recipients who have heart disease are most vulnerable, so their transfusions are given more slowly and they are monitored closely.Graft-versus-host disease is an unusual complication that affects primarily people whose immune system is impaired by drugs or disease. In this disease, the recipient's (host's) tissues are attacked by the donated white blood cells (the graft). The symptoms include fever, rash, low blood pressure, low blood counts, tissue destruction, and shock. These reactions can be fatal but are eliminated by treating with radiation those blood products that are intended for people with a weakened immune system.