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If we collect samples of blood from over ten people and put in bottles that look the same, the blood samples will most likely look the same without any glaring difference, provided all conditions remain the same.
However, if two of these blood samples are mixed, there may be agglutination; that is blood clots begin to form upon standing. The difference in blood groups of these samples is what is responsible for the blood clots observed which were discovered by Karl Landsteiner in 1901.
Red blood cells are biconcave in shape and have a surface membrane. Landsteiner discovered antigens on the surfaces of these red cells which he called agglutinogens because of their ability to cause agglutination; he named them the A and B antigens.
There are over 20 blood groups in existence but only the ABO blood group system is used. Some blood groups are more dominant in some areas and countries.
ABO blood groups are four in number;
Following the discovery by Karl Landsteiner, the blood group is determined based on the agglutination abilities of the antigens present on the surface of the red blood cells. This principle states that “Agglutination occurs if a particular antigen is mixed with its corresponding antibody”.
For example, if antigen A is mixed with antibody A (anti – A) agglutination will occur and blood clots will be seen.
Blood group check is usually done when transfusion is to be done in order to prevent transfusion reactions.
If blood group match is not done before transfusion, some of the reactions and conditions highlighted below may ensue
In addition to the A and B antigens which can be seen on the red blood cells, the D antigen also known as rhesus factor is either present or absent on the surface of red cells.
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When an individual has the Rhesus D factor, it is usually indicated by a + on the blood group, and when the rhesus D antigen is absent, it is indicated by a –
The table below shows what the blood groups of individuals will be depending on the presence or absence of the D antigen (rhesus factor).
Antigen present on surface | Rhesus D antigen | Blood group of individual |
A | Present | A+ |
B | Present | B+ |
A and B | Present | AB+ |
Neither A nor B | Present | O+ |
A | Absent | A- |
B | Absent | B- |
A and B | Absent | AB- |
Neither A nor B | Absent | O- |
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