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Blood transfusion

A transfusion is when blood, or one of the different parts of blood, is given into your bloodstream through a small tube (cannula) put into a vein, usually in your arm. A blood transfusion can be lifesaving and may be used to replace blood that has been lost, for example during an operation or in an accident, or to treat anaemia.

About your blood

Your blood is made up of many different components including:

  • red blood cells – these contain haemoglobin, which carries oxygen around your body
  • white blood cells – these are part of your immune system and are important for fighting infection
  • platelets – these help your blood to clot
  • plasma – this is the fluid that carries cells around your body

You can have a transfusion of red blood cells (this is what is usually called a blood transfusion) or you can have a transfusion of one of the other separate components of blood, such as plasma or platelets.

Why do you need a blood transfusion?

Some of the main reasons why you might need a transfusion are listed below.

  • Severe anaemia. This is when your blood can't carry enough oxygen to meet the needs of your body. This can happen if you have fewer red blood cells than usual, or the amount of haemoglobin in each cell is less than normal.
  • During an operation. If you’re having major surgery, such as heart surgery, you may need a blood transfusion to replace blood that you have lost.
  • After an accident. If you have a serious accident, such as a road traffic accident, you may need a blood transfusion.
  • During pregnancy or birth. Some women bleed heavily after a miscarriage, or during or after labour. If this happens, you may need a blood transfusion.

If you’re worried about having a blood transfusion, discuss it with your doctor.

What are blood groups?

Your red blood cells carry hundreds of different markers on their surface. These are called antigens, and they act as 'identity tags'. What blood group you are depends on which antigens you have on your red blood cells.

There are two blood grouping systems that are the most important for matching your blood with someone else’s blood if you need a transfusion. These are the ABO system and the Rhesus (RhD) system. There are other blood group systems, but these are usually not as important for blood transfusion.

The ABO system

If your blood group is A then you have A antigens (tags) on your red blood cells. Blood group B means you have B antigens. If you’re group AB, then you have some of each antigen and if you’re blood group O, you have neither.

In your blood you have antibodies, which will defend your body against what it sees as foreign antigens. So, for example, if you’re blood group A, you will have anti-B antibodies. This means that if you were given group B blood in a transfusion instead of blood group A, your body’s natural defences would attack the donated blood. This is very serious and is the reason why you will always be given blood that is compatible with your own blood group.

The Rhesus (RhD) system

If your blood group is RhD positive then you have RhD antigen on your red blood cells. RhD negative means you don't have it. About eight out of 10 people have the RhD antigen and are Rhesus positive. About two out of 10 people don’t have the RhD antigen and are Rhesus negative.

If you’re RhD negative you shouldn't be given RhD positive blood because you may develop rhesus antibodies. This is particularly important if you’re a woman who is pregnant or you have the potential to become pregnant. If you’re rhesus negative and your baby is rhesus positive small amounts of blood can cross the placenta from your baby to you. When this happens your body produces antibodies which can destroy your baby’s rhesus positive red cells. This can cause your baby to become anaemic and fluid can leak into his or her tissues which can be very serious. The same problem can also affect any babies you have in the future. This is the reason why you have a blood test when you’re pregnant, to check your rhesus blood group and whether you have RhD antibodies.

Your blood group is made up of your ABO group and your RhD antigen group together. So, for example, you may be A positive, O negative or B negative. Some blood groups are more common than others. Group O positive is the most common group and group AB negative is the least common.

What happens during a blood transfusion?

Before you have any type of blood transfusion you will have a blood sample taken. This is tested to see what blood group you are. Your blood is compared to the donor blood you will be receiving before you have the transfusion. This is to make sure it’s the right blood group and is called cross matching.

Your nurse or doctor will ask you for your name and date of birth and check this against your hospital wrist band if you have one, and against the blood or blood product. This is to make sure you’re having the blood which has been specifically prepared for you.

A cannula is put into a vein, usually in your arm. This is attached by plastic tubing to a bag of blood (called a unit of blood) which hangs on a stand beside you. It usually takes two to four hours to have one unit of blood (about 300 millilitres), although you can have it more quickly if you need it. If you’re having blood products, such as platelets, these are usually given in a shorter time because the volume is less.

While you’re having the transfusion, your nurse will check on you regularly to make sure you’re feeling well, and to measure your blood pressure, temperature and heart rate.

Where does the blood come from?

In the UK, blood is collected from healthy, unpaid volunteers. Many people donate blood regularly. Every donor has his or her ABO and Rhesus group checked when they give blood. If you donate blood, you will be asked to complete a health questionnaire each time, to make sure you’re healthy and able to donate. Every unit of blood that is collected from donors is checked for infectious diseases and stored until it’s needed by the National Blood Service, which is part of the NHS.

What are the alternatives to blood transfusion?

A transfusion of blood from another person (a donor) is usually only given if there is no other alternative, because the risks sometimes outweigh the benefits. Some of the alternatives are listed below.

  • Cell salvage. This is where your blood that is lost during or after your operation is collected, and given back to you (autologous transfusion). You may have all of your blood given back or the blood may be processed and the plasma removed from it.
  • Autologous pre-donation or pre-deposit. This isn’t generally used in the UK but involves collecting some of your own blood a few weeks before a planned operation and then giving this back to you during or after your operation. It can be useful if you have a rare blood group.
  • If you have anaemia, it can be treated by ways other than blood transfusion; for example if you have iron deficiency you can take iron tablets.

If you would like more information or want to discuss the alternatives to transfusion with donor blood, speak to your doctor or nurse looking after you.

What are the risks of having a blood transfusion?

As with every procedure, there are some risks associated with having a blood transfusion. We have not included the chance of these happening as they are specific to you and differ for every person. Ask your doctor to explain how these risks apply to you.


Side-effects are the unwanted but mostly temporary effects you may get after having the procedure.

During a blood transfusion, you may develop a fever, chills, itching or a rash. If this happens, you may be given medicines, such as paracetamol, to lower your temperature, and antihistamine to relieve any itching and a rash. If you do have side-effects your nurse or doctor may slow down your transfusion so that it goes into your body over a longer period of time.

If you start to feel unwell at all during your blood transfusion, tell your nurse or doctor straight away.


Complications are when problems occur during or after the procedure. Most people having a blood transfusion aren’t affected. Serious complications of blood transfusion are very rare. The main ones are listed below.

  • An immediate reaction to the donor blood. This can happen if the wrong blood is given by mistake. The symptoms of a reaction include fever, wheezing, feeling anxious and passing red-coloured urine. If this type of reaction does happen, your doctor or nurse will stop the blood transfusion and you will need emergency treatment.
  • Infection from bacteria in the donor blood. This can happen very quickly. You may have a high fever, a fast heart rate and shivering. If the infection isn’t treated, you may collapse. Your doctor will treat your symptoms and give you antibiotics to treat the infection.
  • Getting a viral infection from the donor blood. Although stringent measures are taken to prevent the transmission of viral infections, such as hepatitis and HIV, the possibility that this could occur can’t be fully ruled out.
  • Too much fluid in your body. This can happen if you have too many transfusions or if they are given too quickly. You may be short of breath with fast breathing and heart rate. If this happens, your doctor will treat your symptoms and give you medicines to help your body get rid of the extra fluid.


Produced by Rebecca Canvin, Bupa Health Information Team, January 2013.

For answers to frequently asked questions on this topic, see FAQs.

For sources and links to further information, see Resources.

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  • This information was published by Bupa's Health Information Team and is based on reputable sources of medical evidence. It has been reviewed by appropriate medical or clinical professionals. Photos are only for illustrative purposes and do not reflect every presentation of a condition. The content is intended only for general information and does not replace the need for personal advice from a qualified health professional. For more details on how we produce our content and its sources, visit the about our health information page.

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