Blood group -Definition, determination and implications

Blood group -Definition, determination and implications

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.

Blood groups

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.

Blood group systems

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;

  • Blood group A which contains A antigen
  • Blood group B which contains B antigen
  • Blood group AB which contains both A and B antigens and
  • Blood group O for red blood cells which have neither A nor B antigens.

How is a person’s blood group determined?

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.

  • For blood group A, clotting will be observed when mixed with antisera A
  • For blood group B, clotting will be observed when the blood is mixed with antisera B
  • For blood group AB, clotting will be observed with both antisera A and antisera B
  • For blood group O, clotting will neither be observed with antisera A or B.

Why is blood grouping important?

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

  • This is the breakdown of blood cells prematurely and in large amounts. When mismatched blood is transfused, it leads to an immune reaction that causes the premature breakdown of the red blood cells that have just been transfused into the patient.
    This destruction causes haemoglobin to be released into the plasma in large amounts which will have other side effects.
  • Jaundice
    In the normal breakdown of red cells, haemoglobin is released and further broken down to haem and globin. Globin is stored and reused while haem is further degraded to porphyrin and iron. This porphyrin is metabolized to release bilirubin which is conjugated in the liver and excreted in the bile.
    Due to the increased haemoglobin in the circulation, bilirubin levels continue to rise to lead to a condition known as jaundice.
  • Cardiac defects.
    The rise in haemoglobin leads to a corresponding increase in viscosity of blood causing the heart to be overworked in circulation; this, in turn, may lead to heart failure.
    Additionally, the increase in toxic substances secreted into the blood may lead to cardiac shock due to increased pressure in arteries.
  • Kidney dysfunction.
    Blockage of the glomerulus by toxic substances leads to kidney failure hence reduces or totally stops the production of urine by the kidney. This is very dangerous as toxic substances continue to accumulate in the body leading to further damage and death if untreated.

Rhesus factor

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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