What kind of blood you have is determined by the genes you inherit from your parents and is divided into different types, known as blood groups. The four main blood groups are A, B, AB and O.

Each group can be either RhD positive or RhD negative, which means that your blood group can be one of eight types (see below).

What is blood?

Your body carries around 4-6 litres (8.5-12.5 pints) of blood. Blood is made up of red blood cells, white blood cells and platelets in a liquid called plasma.

Plasma is about 90% water, but also contains proteins, nutrients, hormones and waste products. Blood is made up of about 60% plasma and 40% blood cells.

Each type of blood cell has a specific role to play:

• red blood cells transport oxygen around the body and remove carbon dioxide and other waste products; they give your blood its red colour
• white blood cells are part of the immune system (the body’s natural defence mechanism) and help fight infection 
• platelets help the blood to clot (thicken) to stop bleeding 

Blood groups

Your blood group is identified by antigens and antibodies present in the blood. Antigens and antibodies are your blood’s natural defences against foreign substances.

Antigens are protein molecules found on the surface of red blood cells. Antibodies are found in plasma. Antibodies recognise anything foreign in your body and alert your immune system so that it can destroy it.

The ABO system

Blood groups are defined by the ABO system.

• blood group A has A antigens on the red blood cells with anti-B antibodies in the plasma
• blood group B has B antigens with anti-A antibodies in the plasma
• blood group O has no antigens but both anti-A and anti-B antibodies in the plasma
• blood group AB has both A and B antigens but no antibodies

Group O is the most common blood group in the UK, with 44% of the population having group O blood (37% O+ and 7% O-).

Receiving blood from the wrong ABO group could be life threatening because antibodies in a person with group A blood will attack group B antigens and vice versa. This will cause a severe reaction in the person receiving the blood.

As group O red blood cells do not have any A or B antigens, it can safely be given to any other group.

The NHS Blood and Transplant website has more information about the different blood groups.

The Rh system

Red blood cells sometimes have another antigen, a protein known as the RhD antigen. If this is present, your blood group is RhD positive. If it is absent, your blood group is 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-)

About 85% of the UK population is RhD positive.

In most cases, O RhD negative blood can safely be given to anyone. However, this depends on specific antibodies and antigens being present in the blood.

Read more about what blood is used for.

Blood group test

To work out your blood group, your red cells are mixed with different antibody solutions. If, for example, the solution contains anti-B antibodies and you have B antigens on your cells, it will clump together.

If the blood does not react to any of the anti-A or anti-B antibodies, it is blood group O. A series of tests with different types of antibody can be used to identify your blood group, including groups other than the main ABO and RhD groups.

If you have a blood transfusion (where blood is taken from one person and given to another), your blood will be tested against a panel of donor cells that contain all of the clinically significant antigens. If there is no reaction, donor blood with the same ABO and RhD type can be used.

Pregnancy

Pregnant women always have a blood group test. This is because if the mother is RhD negative but the child has inherited RhD positive blood from the father, it could cause complications if left untreated.

RhD negative women of child-bearing age should always only receive RhD negative blood.

Donated blood is often used to replace blood lost during surgery or an accident.

In the UK, blood is not used whole but separated into red cells, platelets and plasma so it can be used in various treatments.

However, in cases where a severe amount of blood has been lost, all three blood components can be given in a blood transfusion.

Blood transfusions

A blood transfusion is where blood is taken from one person (a donor) and transferred into another person.

The blood is transferred directly through the blood vessels (intravenously). Some blood groups cannot be mixed with each other, so the blood group you receive must be compatible with your own.

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

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

Your blood group will be identified using a blood group test before a blood transfusion takes place.

How blood is used

Blood is separated into its various parts and each part is used to treat different conditions. There are many conditions treated by donated blood.

Red blood cells

Red blood cells are used to treat all types of anaemia, such as in cancer, for example, or bone marrow disease and sickle cell disease. They can also be used to replace red blood cells lost in accidents, surgery and childbirth.

Platelets

Platelets can be used in conditions that cause the platelet count to be very low, including bone marrow disease and leukaemia (cancer of the white blood cells). They can also be used with chemotherapy and in people who have had a blood stem cell transplant.

Plasma

Plasma can either be frozen or processed for use in a wide variety of treatments.

Frozen plasma is used to replace blood clotting factors when there is massive blood loss; for example, following a severe injury.

It may also be needed for conditions that decrease the production of clotting factors, such as liver disease. A protein called albumin found in plasma can also be used to treat burns.

Processed plasma is important for treating all types of haemophilia (a genetic condition that affects the blood’s ability to clot).

It can also be used to treat pregnant women with RhD negative blood who are carrying a baby with RhD positive blood inherited from the father.

Processed plasma can also be used when producing antibodies that are used to treat conditions such as:

• tetanus 
• hepatitis 
• chickenpox
• rabies