A transfusion reaction results when your body's immune system attacks the blood you received in a blood transfusion or when the transfused blood causes an allergic reaction.

The most serious transfusion reactions occur when your ABO or Rh blood type and that of the transfused blood do not match. These reactions result in the destruction of the transfused blood and can be life-threatening. Less commonly, a mismatch of one of the more than 100 minor blood types can cause a serious reaction.

Mild transfusion reactions usually are caused by a mismatch of the minor blood types between your blood and the transfused blood. A mild reaction may involve fever, shortness of breath, pain, rapid heart rate, chills, and low blood pressure. While a mild transfusion reaction is frightening, it is rarely life-threatening when treated quickly.

Allergic reactions to blood can cause itching, hives, and wheezing. A severe allergic reaction can result in anaphylactic shock, a life-threatening condition.