Blood groups

Page last reviewed: 13/07/2011

Your body carries around four to six litres (8.5 to 12.5 pints) of blood. Blood is made up of red blood cells, white blood cells and platelets in a liquid called plasma.

  • Plasma contains proteins, nutrients, hormones and waste products but mainly consists of water (90%).
  • Red blood cells transport oxygen around the body and remove carbon dioxide and other waste products.
  • White blood cells fight infection.
  • Platelets help the blood to clot.

People can have different blood types, known as blood groups. There are four main blood groups: A, B, AB and O. Each group can then be either RhD positive or negative, so your blood group can be one of eight types.

The genes you inherit from your mother and father determine your blood group.

Types of blood groups

Your blood group is identified by the antigens and antibodies present in the blood. Antigens are usually protein molecules found on the surface of red blood cells. Antibodies are found in the plasma. They are your blood's natural defence against any foreign antigens.

The ABO system

  • Blood group A has A antigens in its red blood cells and anti-B antibodies in its plasma. 
  • Group B has B antigens and anti-A antibodies in its plasma.
  • Group O blood has no antigens but both anti-A and anti-B antibodies. It is the most common blood group in Ireland
  • Group AB has both A and B antigens but no antibodies, as it would destroy itself.

The Rh system

The red blood cells sometimes have another antigen, a protein called the Rh factor. If this antigen is present, your blood group is RhD positive. If it is absent, you are RhD negative. This means that you can be one of eight blood groups:

  • A RhD positive (A+),
  • A RhD negative (A-),
  • B RhD positive (B+),
  • B RhD negative (B-),
  • O RhD positive (O+),
  • O RhD negative (O-),
  • AB RhD positive (AB+),
  • AB RhD negative (AB-).

Around 85% of the Irish population is RhD positive. 


Blood supplies oxygen to the body and removes carbon dioxide. It is pumped around the body by the heart.

Red blood cells

Red blood cells transport oxygen around the body and remove carbon dioxide.

White blood cells

White blood cells fight infection and disease.


Platelets are cells in the blood that control bleeding by plugging the broken blood vessel and helping the blood to clot.


Plasma is the liquid part of blood, which holds other blood cells together.


Oxygen is an odourless, colourless gas that makes up about 20% of the air we breathe.

Page last reviewed: 13/07/2011

Some blood groups cannot be mixed with each other. So if you have a blood transfusion (where blood is taken from one person and put into another), the blood group you receive must be compatible with your own.

For example, if you are blood group A, you cannot take blood from a person with B blood as the anti-B antibodies in your blood will fight the B antigens in the donor's blood. This causes the transfused red cells to be destroyed, which can be fatal.

Blood group O- has no ABO or RhD antigens and can be given to almost anyone. Someone with blood group AB+ can usually receive blood from any group, but AB blood can only be given to a person with blood group AB.

Blood group test

To work out your blood group, your blood is mixed with different samples of serum (blood without the clotting agent), which is already known to contain different antibodies. If the serum contains anti-B antibodies and you have B antigens on your blood it will clump together, meaning that the antibodies are fighting the antigens. If the blood does not react to any of the antibodies, it is blood group O. A series of tests with different types of serum will identify your blood group.

If you have a blood transfusion, your blood will always be mixed with a sample of the donor's blood to check for clumping, even if your blood groups are compatible. This is because there are many other types of antigens in the blood, which will not always be compatible (although this is rare).


A blood group test will always be done on a pregnant woman. If she is RhD negative but the child has inherited RhD positive blood from the father, it could cause complications if left untreated.


Antibodies and immunoglobins are proteins in the blood. They are produced by the immune system to fight against bacteria, viruses and disease.
Blood transfusion
A blood transfusion involves transferring blood into a person using a tube that goes directly into a vein in the arm.
Blood supplies oxygen to the body and removes carbon dioxide. It is pumped around the body by the heart.
A donor is a person (living or dead) who donates blood, an organ or other body parts to another person in need.

Content provided by NHS Choices and adapted for Ireland by the Health A-Z.

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