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Blood Type Test

It is essential that everyone know their own blood group and that of their family members. There are many occasions when knowing your blood group could be important, for example, if you or a member of your family, required an urgent blood transfusion it would be useful to know your blood group in advance. If it was an uncommon group it would allow the hospital valuable time to ensure adequate supplies.

If you travel overseas carrying a blood group card could prove invaluable. Accidents and other emergencies can often be dealt with more efficiently knowing the blood group of the person or persons involved.

Before blood transfusion a sample of blood collected from the donor must be tested. The first area of testing relates to determining the ABO group and the Rh type of the donor's blood.

Human blood is typed, according to the presence or absence of certain antigens the red blood cells. blood type test may also be done to see if two people are likely to be blood relatives. The most important antigens are blood group antigens (ABO) and the Rh antigen. Therefore, the two most common blood type tests are the ABO and Rh tests.

ABO test

The ABO test classifies people's blood into one of four types: A, B, AB, or O. If your red blood cells have:

  • The A antigen, you have type A blood. The liquid portion of your blood (plasma) contains antibodies against type B blood.
  • The B antigen, you have type B blood. Your plasma contains antibodies against type A blood.
  • Neither the A nor B antigen, you have type O blood. Your plasma contains antibodies against both type A and type B blood.
  • Both the A and B antigens, you have type AB blood. Your plasma does not contain antibodies against either type A or type B blood

Blood received in a transfusion must contain antigens that are the same as the antigens on a person's own red blood cells (compatible blood). If you receive a transfusion that contains antigens different from your own (incompatible blood), the antibodies in your plasma will recognize the transfused (donor) blood as foreign and will attack and destroy the donor red blood cells. This is called a transfusion reaction, and it occurs immediately when incompatible blood is transfused. A transfusion reaction can be mild or cause a serious illness and even death.
Type O-negative blood does not have any antigens. It is called the "universal donor" type because it is compatible with any blood type. Type AB-positive blood is called the "universal recipient" type because a person who has it can receive blood of any type. Although “universal donor” and “universal recipient” types are occasionally used to classify blood in an emergency, blood type tests are always done to prevent transfusion reactions.

Minor antigens (other than A, B, and Rh) that occur on red blood cells can sometimes also cause problems and so are also checked for a match before giving a blood transfusion.

Serious transfusion reactions are rare today because of blood type tests.

Rh test

Rh blood type checks for the presence (+) or absence (–) of the Rh antigen (also called the Rh factor). If your red blood cells:

  • Contain the Rh antigen, your blood is Rh-positive.
  • Do not contain the Rh antigen, your blood is Rh-negative.
  • Contain the A and Rh antigens, your blood type is A-positive (A+). If your blood contains the B antigen but not the Rh antigen, your blood type is B-negative (B–).

Rh blood type is especially important for pregnant women. A potential problem arises when a woman who has Rh-negative blood becomes pregnant with a baby (fetus) that has Rh-positive blood. This is called Rh incompatibility. If the blood of an Rh-positive baby mixes with the blood of an Rh-negative woman during pregnancy or delivery, the mother's immune system produces antibodies. This antibody response is called Rh sensitization and, depending on when it occurs, can destroy the baby's red blood cells.

Rh sensitization does not usually affect the health of the baby during the pregnancy in which the sensitization occurs. However, the baby of a future pregnancy is more likely to be affected if the baby's blood type is Rh-positive. Once sensitization has occurred, the baby can develop mild to severe problems (called Rh disease, hemolytic disease of the newborn). If untreated, complications from sensitization can, in rare cases, lead to the death of an Rh-positive baby.

An Rh test is done in early pregnancy to detect a woman's blood type. If she is Rh-negative, she can receive an antibody injection called Rh immune globulin (such as RhoGAM) that almost always prevents sensitization from occurring. Problems arising from Rh sensitization have become very rare since the Rh immune globulin injection was developed.