Your blood is typed, or classified, according to the presence or absence of certain markers (antigens) found on red blood cells and in the plasma that allow your body to recognize blood as its own. If another blood type is introduced, your immune system recognizes it as foreign and attacks it, resulting in a transfusion reaction.
ABO blood type system
The ABO system consists of A, B, AB, and O blood types. People with type A have antibodies in the blood against type B. People with type B have antibodies in the blood against type A. People with AB have no anti-A or anti-B antibodies. People with type O have both anti-A and anti-B antibodies. People with type AB blood are called universal recipients, because they can receive any of the ABO types. People with type O blood are called universal donors, because their blood can be given to people with any of the ABO types. Mismatches with the ABO and Rh blood types are responsible for the most serious, sometimes life-threatening, transfusion reactions. But these types of reactions are rare.
For every 1 million units of blood transfused, getting the wrong blood type happens, at the most, 4 times.1 Transfusion with the wrong blood type can cause a severe reaction that may be life-threatening.2
The Rh system classifies blood as Rh-positive or Rh-negative, based on the presence or absence of Rh antibodies in the blood. People with Rh-positive blood can receive Rh-negative blood, but people with Rh-negative blood will have a transfusion reaction if they receive Rh-positive blood. Transfusion reactions caused by mismatched Rh blood types can be serious.
Minor blood types
There are over 100 other blood subtypes. Most have little or no effect on blood transfusions, but a few of them may be the main causes of mild transfusion reactions. Mild transfusion reactions are frightening, but they are rarely life-threatening when treated quickly.