The main reasons to know your blood group are if you need to have a blood transfusion or if you are pregnant.
What is normal blood made up of?
These can be seen under a microscope and make up about 40% of the blood's volume. Blood cells are divided into three main types:
- Red cells (erythrocytes). These make blood a red colour. One drop of blood contains about five million red cells. A constant new supply of red blood cells is needed to replace old cells that break down. Millions of red blood cells are made each day. Red cells contain a chemical called haemoglobin. This binds to oxygen and takes oxygen from the lungs to all parts of the body.
- White cells (leukocytes). There are different types of white cells which are called neutrophils (polymorphs), lymphocytes, eosinophils, monocytes and basophils. They are part of the immune system. Their main role is to defend the body against infection. Neutrophils engulf germs (bacteria) and destroy them with special chemicals. Eosinophils and monocytes also work by swallowing up foreign particles in the body. Basophils help to intensify inflammation. Inflammation makes blood vessels leaky. This helps specialised white blood cells get to where they are needed. Lymphocytes have a variety of different functions. They attack viruses and other germs (pathogens). They also make antibodies which help to destroy pathogens.
- Platelets. These are tiny and help the blood to clot if we cut ourselves.
This is the liquid part of blood and makes up about 60% of the blood's volume. Plasma is mainly made from water but also contains many proteins and chemicals. Antibodies are one of the types of protein found in plasma.
What is a blood group?
Red blood cells (erythrocytes) have certain proteins on their surface, called antigens. Also, your plasma contains antibodies which will attack certain antigens if they are present. ABO and rhesus are both types of antigens found on the surface of red blood cells. There are lots of other types but these are the most important.
These were the first type discovered.
- If you have type A antigens on the surface of your red blood cells, you also have anti-B antibodies in your plasma.
- If you have type B antigens on the surface of your red blood cells, you also have anti-A antibodies in your plasma.
- If you have type A and type B antigens on the surface of your red blood cells, you do not have antibodies to A or B antigens in your plasma.
- If you have neither type A nor type B antigens on the surface of your red blood cells, you have anti-A and anti-B antibodies in your plasma.
It is not known what the functions of the A and B antigens are. People who don't have either (blood group O) are still just as healthy. There is some evidence that people of different blood groups may be more or less susceptible to certain diseases - for example, blood clots in the blood vessels (thromboembolism) and malaria. There is no evidence that people with different blood groups should follow different diets.
Most people are 'rhesus positive'. This means they have rhesus antigens on their red blood cells. But, about 3 in 20 people do not have rhesus antibodies and are said to be 'rhesus negative'.
Blood group names
Your blood group depends on which antigens occur on the surface of your red blood cells. Your genetic make-up, which you inherit from your parents, determines which antigens are present on your red blood cells. Your blood group is said to be:
- A+ (A positive) if you have A and rhesus antigens.
- A− (A negative) if you have A antigens but don't have rhesus antigens.
- B+ (B positive) if you have B and rhesus antigens.
- B− (B negative) if you have B antigens but don't have rhesus antigens.
- AB+ (AB positive) if you have A, B and rhesus antigens.
- AB− (AB negative) if you have A and B antigens but don't have rhesus antigens.
- O+ (O positive) if you have neither A nor B antigens but you have rhesus antigens.
- O− (O negative) if you don't have A, B or rhesus antigens.
Other blood types
There are many other types of antigen which may occur on the surface of red blood cells. However, most are classed as 'minor' and are not as important as ABO and rhesus.
How is blood group testing done?
Basically, a sample of your blood is mixed with different samples of plasma known to contain different antibodies. For example, if plasma which contains anti-A antibodies makes the red cells in your blood (erythrocytes) clump together, you have A antigens on your blood cells. Or, if plasma which contains rhesus antibodies makes the red cells in your blood clump together, you have rhesus antigens on your blood cells. By doing a series of such tests it is possible to determine what antigens are on your red blood cells and therefore determine your blood group.
Routine blood grouping checks for your ABO and rhesus status. Other red cell antigens are tested for in certain other situations.
Blood transfusions and cross-matching
If you have a blood transfusion, it is vital that the blood you receive is well matched (compatible) with your own. For example, if you receive blood from a person who is A positive and you are B positive, then the anti-A antibodies in your plasma will attack the red blood cells (erythrocytes) of the donated blood. This causes the red cells of the donated blood to clump together. This can cause a serious or even fatal reaction in your body.
Therefore, before a blood transfusion is done, a donor bag of blood is selected with the same ABO and rhesus blood group as yourself. Then, to make sure there is no incompatibility, a small sample of your blood is mixed with a small sample of the donor blood. After a short time the mixed blood is looked at under a microscope to see if there has been any clumping of blood. If there is no clumping, then it is safe to transfuse the blood.
Blood groups and pregnancy
A blood group test is always done on pregnant women. If the mother is rhesus negative and the unborn baby is rhesus positive (inherited from a rhesus positive father) then the mother's immune system may produce anti-rhesus antibodies. These may attack and destroy the baby's blood cells. This is rarely a problem in a first pregnancy. However, without treatment, this can become a serious problem in subsequent pregnancies, as the mother's immune system will be 'sensitised' after the first pregnancy.
Giving blood is simple and saves lives. The blood transfusion service needs people of all blood groups to donate blood, but especially if you have one of the rarer blood groups. By donating blood, you will find out what blood group you are (see below).
Further help & information
Dr Tim Kenny
Dr Jacqueline Payne
Dr John Cox