Transfusion Reaction

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Overview, Causes, & Risk Factors

The immune response normally protects the body from potentially harmful substances. These substances (antigens) trigger multiple responses, including production of antibodies (immunoglobulins, molecules that attach to a specific antigen and aid in its destruction), and sensitized lymphocytes that recognize a particular antigen and destroy it.

The immune system normally can distinguish its own blood cells from other cells. These foreign proteins (antigens) produce an immune response.

The surface of red blood cells contain several proteins that can be identified by the body as antigens. In 1900, the German pathologist, Karl Landsteiner, identified 2 of these antigenic proteins, which he called A and B.

Blood is classified according to the presence of these antigens, resulting in blood types A, B, AB (contains both antigens), and O (contains neither antigen). Blood plasma contains antibodies against the opposite antigen. A person with Type A blood, for example, has antibodies against the B antigen.

In 1940, Dr. Landsteiner discovered another group of antigens. They were named Rhesus factors (Rh factors) because they were discovered during experiments on Rhesus monkeys.

People with Rhesus factors in their blood are classified as "Rh positive," while persons without the factors are classified as "Rh negative." Rh negative persons form antibodies against the Rh factor if they are exposed to Rh positive blood.

This is of major importance in an Rh negative mother who is pregnant with an Rh positive baby. There are other antigens as well, besides ABO and Rh antigens.

The presence of antibodies against blood antigens results in blood group compatibility or incompatibility. Transfusion of blood between compatible groups (such as O+ to O+) usually causes no problem. Blood transfusion between incompatible groups (such as A+ to O-) causes an immune response against the cells carrying the antigen, resulting in transfusion reaction.

The immune system attacks the donated blood cells, causing them to burst. This may cause serious symptoms, including kidney failure and shock. Antigens also occur on other blood components, including white blood cells, platelets, and plasma proteins.

These components also cause a similar type of transfusion reaction. Alternatively, antibodies in the transfused blood can bind the patient's own blood cells, also causing a reaction.

Today, all blood is carefully screened. Modern lab methods and redundant checks have helped make transfusion reactions extremely rare.