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Blood Test – Blood Grouping

What is blood made up of?

  • Plasma, the liquid part of blood, makes up about 60% of the blood’s volume. Plasma is mainly made from water, but contains many different proteins and other chemicals such as hormones, antibodies, enzymes, glucose, fat particles, salts, etc.
  • Blood cells, which can be seen under a microscope, make up about 40% of the blood’s volume. Blood cells are made in the bone marrow by blood ‘stem’ cells. 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 are released into the bloodstream from the bone marrow 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 (leucocytes). There are different types such as neutrophils (polymorphs), lymphocytes, eosinophils, monocytes, basophils. They are a part of the immune system and are mainly involved in combating infection.
    • Platelets. These are tiny and help the blood to clot if we cut ourselves.

What is a blood group?

Red blood cells have certain proteins on their surface, called antigens. Also, your plasma contains antibodies which will attack certain antigens if they are present. There are various types of red blood cell antigens – the ABO and rhesus types are the most important.

ABO types

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.

Rhesus types

Most people are ‘rhesus positive’ as 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 occur on your red blood cells. Because of this fact a blood group test is sometimes used to help settle disputes about who is the father of a child. 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 not rhesus antigens.
  • B+ (B positive) if you have B and rhesus antigens.
  • B– (B negative) if you have B antigens, but not rhesus antigens.
  • AB+ (AB positive) if you have A, B and rhesus antigens.
  • AB– (AB negative) if you have A and B antigens, but not rhesus antigens.
  • O+ (O positive) if you have neither A nor B antigens, but you have rhesus antigens.
  • O– (O negative) if you have do not 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 clump together, then you have A antigens on your blood cells. Or, if plasma which contains rhesus antibodies makes the red cells in your blood clump together, then 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 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 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.

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