Blood typing
How is blood classified?
Blood is classified according to the nature of the chemical substances known as antigens or markers, which are microscopic substances found on the surface of red blood cells.
Although there are many blood group systems with several sub-types, the two best-known ways of classifying blood are the ABO group system and the rhesus (Rh) type system.
Antigens and antibodies
The role of antigens is to help the body recognise foreign substances introduced into the body. If this happens, for example, through an incompatible blood transfusion, the antigens recognise the foreign substances. The body then releases chemicals known as antibodies, which are found in the plasma (the watery part of the blood), as protection. These antibodies combine with and neutralise these foreign cells, producing immunity against them.
The ABO group system
In 1901, Austrian pathologist Karl Landsteiner discovered there were 2 kinds of antigens on the surface of red blood cells. He called these antigens A and B. A year later, 2 other researchers discovered that some people had both A and B antigens in their blood. This formed the basis for the ABO group system of classification. So, depending on whether a person’s blood contains one or the other antigen, both, or neither, their blood is classified as type A, B, AB, or O respectively.
Under the ABO system, the absence of a particular antigen corresponds to the presence of an antibody for that antigen.
- If a person is blood group A, they will have A antigens on their red blood cells and they will have antibodies to antigen B in their plasma.
- Likewise, if they are blood group B, they will have B antigens on their red blood cells and antibodies to antigen A in their plasma.
- People with blood type AB have both A and B antigens on their red blood cells, so they do not have antibodies to either A or B in their plasma.
- People with blood type O, who have no A or B antigens, have antibodies to both A and B in their plasma.
The rhesus (Rh) system
Another antigen known as the rhesus factor (abbreviated as Rh factor) is found on the surfaces of red blood cells. Named because it was first discovered in rhesus monkeys, it is present in some people’s blood but not in others. Those who have it are said to be Rh positive, those who do not have it are said to be Rh negative. One difference between the Rh system and the ABO group system is that Rh negative people don't usually possess Rh antibodies, whereas in the ABO group system if the antigen is absent, the antibody to it is present.
In any of the 4 ABO groups, a person can be Rh positive or Rh negative, meaning that a person’s blood can be classified as one of 8 possible types (O+, O-, A+, A-, B+, B-, AB+, AB-). The blood group found most often in the Australian population is O positive, with 40 per cent of people possessing this blood type. On the other hand, only 1 per cent of Australians are AB negative.
Why do we need to classify blood?
Classifying blood types is important in blood transfusions. A successful transfusion depends on the blood of the donor and the receiver (the recipient) being compatible. If it is not, illness and even death may result for the person receiving the donated blood. A person who needs a transfusion cannot safely receive blood of an ABO group containing antigens to which he or she has antibodies.
So, for example, if a person with type A blood (whose plasma contains antibodies to B) is transfused with blood from a type B donor (whose plasma has antibodies to A), a transfusion reaction will occur. This could cause the destruction of blood cells, causing shortness of breath, severe headache, chest or back pain and blood in the urine. Other symptoms of a transfusion reaction may be fever, chills, itching, hives, or aches and pains. That is why cross matching of blood is absolutely necessary before a transfusion is performed.
| 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 |
| O | O |
In addition, people who are Rh negative should, in general, only be given Rh negative blood (as it contains no Rh antibodies). However, those who are Rh positive can receive either Rh positive or Rh negative blood.
Universal donors and receivers
In an emergency, individuals who are O Rh negative can donate red blood cells to individuals of any blood type, and those who are AB Rh positive can receive red blood cells of any ABO type. Therefore, people with type O- blood are known as ‘universal donors’ while those with type AB+ blood are known as ‘universal recipients’. In addition, people who are AB- can donate their plasma to anyone. However, doctors generally prefer to give people blood of the same type as their own, if it is possible.
Last Reviewed: 16 September 2009
- 1. Basics of red cell immunology and compatibility testing. In: Handbook for transfusion medicine. 4th ed. Norwich: UK; 2007. Available from: http://www.transfusionguidelines.org.uk/index.asp?Publication=HTM&Section=9&pageid=1111 (accessed Nov 2009).
2. National Heart Lung and Blood Institute. Diseases and conditions index. Blood transfusions [website]. Updated July 2009. Available from: http://www.nhlbi.nih.gov/health/dci/Diseases/bt/bt_whatis.html (accessed Nov 2009).
Tiada ulasan:
Catat Ulasan