Blood typing

by | Tests and Investigations

What is blood typing?

Blood typing is a test done to work out which blood type you have, or which blood group you belong to. Not everyone has the same blood group. There are several different blood groups, some of which are more common than others.

People who have matching blood groups are said to be ‘compatible’. This means they could give or receive each other’s blood if necessary.

Why is blood typing done?

Blood typing is done whenever there is a need to know which blood group you have. For example:

  • if you are pregnant – to make sure your blood and the blood of your baby are compatible
  • if you need a blood transfusion or organ transplant – to make sure the blood you receive from a donor is compatible with your blood
  • if you want to donate blood – so doctors know who they can safely give your blood to

Blood typing is also sometimes done in newborn babies to test for certain diseases and conditions.

How is blood classified?

Blood is classified into blood groups according to whether certain substances are present or not. These include antigens (types of sugars and proteins) found on the surface of your red blood cells and antibodies (types of protein) which are mainly found in plasma – the liquid component of your blood.

All cells have different combinations of markers or “antigens” on their surface. Our immune system has learned to use these to help distinguish the body’s own cells (self) from foreign bodies (non-self), like bacteria or toxins. Your immune system learns to ignore your own normal antigens, but when it recognises that a foreign antigen has entered the body, it releases antibodies, which attach to the foreign entity and mark it so other parts of the immune system can remove and destroy it.

In the case of blood cells, if your immune system recognises that foreign blood cells (i.e. blood cells that are not your type) have been introduced into your body (such as via a transfusion), it will mount an immune response using antibodies against the foreign cells.

The most common systems used for classifying blood are the ABO blood group system and the Rhesus (Rh) type system.

The ABO blood group system

Two of the antigens found on the surface of red blood cells are called antigen A and antigen B. The ABO group system for blood typing is based on which of these antigens you have on your red blood cells. You inherit the particular combination of blood group antigens you have from your parents.

  • Blood type A: your red blood cells have antigen A only.
  • Blood type B: your red blood cells have antigen B only.
  • Blood type AB: your red blood cells have both A and B antigens.
  • Blood type O: you have neither A nor B antigens on your red blood cells.

In your plasma (the liquid component of your blood), you will have antibodies against whichever antigens you don’t have on your own red blood cells i.e. antigens that are foreign. These antibodies are present without you having contact with the “foreign” blood type. How this occurs is not clear yet.

  • If you are blood group A, you will have antibodies to antigen B.
  • If you are blood group B, you will have antibodies to antigen A.
  • If you are blood type AB, you will not have antibodies to either A or B.
  • If you are blood type O, you will have antibodies to both A and B.

The rhesus (Rh) system

The other blood typing system commonly used is the Rhesus system, also called Rh system, named after the Rhesus monkey in which it was first discovered. In this system, if you have an antigen called the RhD antigen on the surface of your red blood cells, you are said to be Rhesus positive (Rh+). If you don’t, you are said to be Rhesus negative (Rh-). In Australia, about 83% of people are Rh positive.

Combining your ABO blood group with whether you are Rh+ or Rh- means your blood can be classified as one of 8 possible types:

  • O positive (O+)
  • O negative (O-)
  • A positive (A+)
  • A negative (A-)
  • B positive (B+)
  • B negative (B-)
  • AB positive (AB+)
  • AB negative (AB-)

The most common blood group in the Australian population is O positive, with about 40 per cent of people having this blood type. On the other hand, only about 1 per cent of Australians are AB negative – the least common type.

One difference between the Rhesus system and the ABO group system is that Rh negative people don’t usually possess antibodies against RhD (unless they have been previously exposed to it), whereas in the ABO group system if the antigen is absent from the red blood cell, the antibody against it is present in the plasma.

Testing to see if you are Rh positive or Rh negative is routinely done during pregnancy, and for blood donors and for people receiving a blood transfusion.

If a mother is Rh negative but her baby is Rh positive (which can happen if the father is Rh positive), the mother could produce antibodies that fight the baby’s red blood cells. This can happen if blood from the unborn baby enters the mother’s circulation. When there is a risk of this happening (threatened miscarriage, termination, chorionic villus sampling (CVS), abdominal trauma, at delivery), an injection called anti-D can be given to the mother to help prevent these antibodies against Rh positive blood being produced. According to the Australian Red Cross Blood Service 17% of Australian mothers will need injections of anti-D.

Universal donors and recipients

It is vital that any blood you receive from a donor is compatible with your own blood. If it is not, you can get very sick or even die. If you need blood, ideally it will come from a donor who is the same ABO and Rh type as you. However, if it’s an emergency and an exact match isn’t available, other types of blood may be compatible – meaning that your immune system won’t react against them.

The table below shows which blood types are compatible.

Compatibility of ABO blood types
ABO blood type of recipient ABO blood types recipient can receive
A A, O
B B, O
AB A, B, AB, O

Universal red cell donors: People with O negative blood don’t have any A, B or Rh antigens on their red blood cells, which means they can donate red blood cells to anyone (their blood cells won’t trigger the recipient’s immune system to “fight” the blood). For this reason, people with O negative blood are referred to as ‘universal donors’.

Universal recipients: People with AB blood group have both A and B antigens on their red blood cells and don’t have antibodies against A or B antigens, which means they can receive red blood cells of any type (their immune system won’t fight them). For this reason, they are referred to as ‘universal recipients.’

Universal plasma donor: In addition, people who are blood group AB can donate their plasma to anyone (because it doesn’t have any antibodies to other blood groups).

In general, people who are Rh negative should only be given Rh negative blood (as it contains no RhD antigens). If they are given Rh positive blood (which does carry the RhD antigen), their immune system will see it as foreign (non-self) and start producing antibodies against the RhD antigen.

People who are Rh positive can receive either Rh positive or Rh negative blood.

How is blood typing done?

Testing to work out which ABO blood group you are in is called ABO blood typing, and involves 2 steps.

Step 1 (called ‘forward typing’)

A sample of your blood is tested separately against 2 laboratory solutions – one that contains antibodies against the A antigen (anti-A) and one that contains antibodies against the B antigen (anti-B).

The way your blood reacts to the antibodies shows which antigens your blood contains. For example, if you have the A antigen on your red blood cells (you are blood group A), when the solution containing anti-A antibodies is added, your blood cells will react by clumping together (agglutinating). If you blood doesn’t react to either of the solutions, it must be O type blood.

Step 2 (‘reverse typing’)

In step 2 (‘reverse typing’), your plasma (the fluid part of your blood after the red and white cells have been removed) is mixed with blood known to be type A, and separately with blood known to be type B, to check its reaction. Whether or not agglutination takes place should confirm the results of the forward typing. Both types of test must agree before your blood type is confirmed.

Rhesus typing

The test to work out which type Rhesus blood type you are is called RhD typing. In this test, a sample of your red blood cells is mixed with a laboratory solution that contains antibodies to RhD. If your blood reacts to the Rh test by agglutinating (clumping), it’s Rh positive. If it doesn’t react, it’s Rh negative.

Both ABO and RhD blood typing are done on all blood collected from donors, as well as on all blood used for transfusions in hospitals. Very occasionally testing is required for other red cell antigens, usually only in people with rare blood disorders.