What are the risks of an Rh mismatch in a transfusion?

Q.What happens when a patient with Rh-negative blood is given Rh-positive blood?

A. Your question relates to the issue of blood groups and making sure that the donation of blood from one person (the donor) to another (the recipient) is safe. The Rh system is one of the two major systems that characterize a person's blood or blood group. (It does not really apply to platelets.) The other major system is the ABO system. These systems refer to the chemical composition of the surface of the red blood cells.

A person is either Rh-positive or Rh-negative. Most people are Rh-positive. An Rh-positive person possesses the so-called Rh factor on their blood cells. If an Rh-negative person receives blood that contains the Rh factor, the Rh-negative patient may develop antibodies (chemicals that react) against it. This may not cause problems unless the person at some later time receives another Rh-positive blood transfusion. Then, the antibodies to the Rh factor (previously produced) may "attack" the newly transfused Rh-positive blood and cause a transfusion reaction, which can be very serious.

Mothers who are Rh-negative and give birth to an Rh-positive baby can also be exposed to the Rh-positive baby's blood during pregnancy. If the woman becomes pregnant again with another Rh-positive baby, a significant reaction can occur. Thus, good medical practice dictates that the blood types of the mother and baby be determined, and specialized medicines can be given to prevent this type of transfusion reaction.

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