Blood carries oxygen and nutrients to every cell in the body and picks up waste products, such as carbon dioxide, for removal. Most 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 circulatory system consists of the heart, blood vessels and blood.
How blood groups are classified#
The two main ways to classify blood groups are the ABO system and the Rh system. Together, they make up the eight main blood groups.
The four 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 inherited from each parent. Each blood group is identified by its own set of molecules, called antigens, which sit 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.
The Rh blood factor#
The Rh type was once called the “Rhesus type”, but we now say “Rh type”. Your Rh type is determined by a different pair of genes from those that decide 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 can have a severe immune-system reaction if Rh-positive blood enters their bloodstream. A person’s blood group is therefore described by the appropriate letter (A, B, AB or O) and by whether the blood is Rh-positive or Rh-negative.
How common each group is#
The frequency of each blood group varies between populations. As an example, one distribution is:
- O positive – about 40%
- O negative – about 9%
- A positive – about 31%
- A negative – about 7%
- B positive – about 8%
- B negative – about 2%
- AB positive – about 2%
- AB negative – about 1%
Blood transfusion#
Transfusions may involve 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. Where blood supplies are well regulated, transfusion 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 cross the placenta, the woman’s immune system may see the Rh-positive cells as a threat. Specialised white blood cells then make antibodies designed to destroy Rh-positive blood cells.
If the woman later conceives another Rh-positive baby, her immune system can flood the fetus with antibodies that destroy the baby’s red blood cells. Left untreated, this can cause severe anaemia or even death of the baby. This is called haemolytic disease of the newborn (HDN).
Preventing haemolytic disease of the newborn#
HDN is now rare, because Rh-negative mothers can be immunised during 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 are not limited to the commonly recognised groups such as A, B and O. Researchers have identified more than 300 other variants, with new antigens still being discovered. Each variant is a marker, or antigen, on the surface of red blood cells.
Some patients need specially matched red cells for transfusion. This means that, on top of being matched by ABO and Rh type, the donor’s blood is matched to make sure it does not contain blood group variants that are not present in the recipient’s blood. As the diversity of a population changes, so too does the demand for these rarer variants.
Key points#
- The two main blood group systems are ABO (A, B, AB and O) and Rh (positive or negative), together forming the eight main blood groups.
- Your blood group is inherited, with one gene from each parent.
- Donated blood must match the recipient’s blood to avoid complications; O negative can be given to anyone if needed.
- An Rh-negative mother carrying an Rh-positive baby may need immunoglobulin to prevent haemolytic disease of the newborn.
- More than 300 minor blood group variants exist, and some patients need specially matched blood.
Where to get help#
Sources & further reading
For evidence-based global guidance on this topic, consult authoritative public-health bodies such as the World Health Organization (WHO), CDC, NHS, and ECDC.