Summary
Read the full fact sheet- The 2 major classifications of blood are the ABO system and the Rh system.
- The 4 blood groups are A, B, AB and O. Each of these will be either Rh-positive or Rh-negative.
- In an emergency, a transfusion of O negative red blood cells can be given to anybody, but it is better to match the exact blood group to avoid serious reactions.
On this page
About blood
The circulatory system consists of the heart, blood vessels and blood. The blood carries oxygen and nutrients to every cell in the body and picks up waste products (such as carbon dioxide) for removal from the body.
The bulk of your blood is made up of plasma. Floating in the plasma are the red blood cells that carry oxygen, the white cells that form part of the immune system, and clotting cells called platelets.
The 2 main ways to classify blood groups are the ABO system and the Rh system. Together, they make up the 8 main blood groups. Other blood group systems exist - to date, researchers have identified more than 300 minor blood groups with new antigens still being discovered.
ABO blood group
The 4 different blood groups in the ABO system are A, B, AB and O. A person's blood group is determined by a pair of genes - one gene inherited from each parent.
Each blood group is identified by its own set of molecules (called antigens), which are located on the surface of red blood cells. When a person needs a blood transfusion, the donated blood must match the recipient's blood or complications will occur.
Rh type blood factor
A person's blood type used to be called their 'Rhesus type' but now we say 'Rh type'. Your Rh type is determined by a different pair of genes to the ones that determine your ABO blood type (again, one inherited from each parent).
Blood is either Rh-positive or Rh-negative, depending on whether certain molecules are present. A person who is Rh-negative will experience a severe immune-system reaction if Rh-positive blood gets into their bloodstream.
Blood groups in Australia
A person's blood group is described by the appropriate letter (A, B, AB or O) and by whether their blood is Rh-positive or Rh-negative.
According to Australian Red Cross Lifeblood, the percentage of blood group frequency in Australia is:
- O positive - 40%
- O negative - 9%
- A positive - 31%
- A negative - 7%
- B positive - 8%
- B negative - 2%
- AB positive - 2%
- AB negative - 1%.
Blood transfusion
A blood transfusion is the transfer of blood or blood components from one person to another. Transfusions are of red blood cells or other components such as plasma or platelets.
O negative red blood cells can be given to anybody if necessary, but it is always preferable to match the exact blood group. Australia has one of the safest blood supplies in the world, and donating blood here is a very safe process.
Rh blood factor and pregnancy
Problems can occur during pregnancy if an Rh-negative woman carries an Rh-positive baby. If blood cells from the baby travel across the placenta, the woman's immune system will see the Rh-positive cells as a threat. Specialised white blood cells will make antibodies designed to kill Rh-positive blood cells.
If the woman later conceives another Rh-positive baby, her immune system will flood the fetus with antibodies. These antibodies then destroy the baby's red blood cells. If left untreated, this can result in severe anaemia or even death of the baby. This is called haemolytic disease of the newborn (HDN).
Preventing haemolytic disease of the newborn (HDN)
HDN is now rare, since Rh-negative mothers are immunised throughout their pregnancy and within 72 hours of giving birth, using an immunoglobulin made from donated blood products. The immunoglobulin breaks down the baby's red blood cells inside the mother's bloodstream before her immune system has time to react.
Rare blood types
Blood types consist not only of the commonly recognised groups such as A, B and O, but also include more than 300 other variants. Each of these variants is a marker on the surface of our red blood cells, and is known as an 'antigen'.
Some patients need specially matched red cells for transfusion. This means on top of being matched by ABO and Rh type, the donor's blood is matched to make sure it doesn't contain blood group variants that aren't present in the recipient's blood. As Australia's ethnic diversity continues to change, so to does the demand for these variants.
Where to get help
- Walter AW 2020, 'Hemolytic disease of the newborn', MSD Manual: consumer version.
- Dulay AT 2020, 'Rh incompatibility', MSD Manual: consumer version.
- Dulay AT 2020, 'Erythroblastosis fetalis', MSD Manual: professionals version
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