Nov 5, 2015

Blood Group – The ABO System

A well-known example of multiple allele is the ABO blood grouping system in human. It was discovered by Karl Landsteiner in 1901. An antigen is usually a foreign substance that enters the body. Antibody is a protein that has been formed in response to antigen. The red blood cells have antigen. The plasma has antibodies. Two antigens A and B and corresponding antibodies have been distinguished. Sometimes clumping occurs when red blood cells of one person are mixed with the blood plasma of another person. On the basis of the reactions mankind are divided into four groups, A, B, AB, O. Some people were found to have ‘A’ antigen some had B and some had both A and B and some had neither A nor B antigen. Those with A type blood did not carry the corresponding anti A antibody, but they did carry anti-B in their plasma. B type people carry anti A but not B. Persons with AB type blood have both A and B antigens associated with the red blood cells, but no Anti A or Anti B antibodies in their plasma. O type individuals have no A and B antigens but carry both anti A and Anti B antibodies in the plasma.

Multiple Alleles for the ABO blood group 
When two different blood groups are mixed, antigen of one reacts with the antibodies (agglutinin which acts as an antibody) of the other, then the RBC clump with one another. The clumping of RBC is known as agglutination.

Genotype of ABO Blood Group
In 1925 Berstein explained the genetic basis of ABO system. Blood group is controlled by autosomal single polymorphic gene I on chromosome. The gene locus (place) is represented by the symbol I (which stands for isohaemagglutinogen). There are three alleles represented by the symbol A, B (these letters refer to two carbohydrates designated A and B which are found on the surface of RBC) and i. The alleles A and B are equally dominant so they are called co-dominant and i is recessive to both A and B.

The genotype I^i would give rise to the agglutinogen A on the red blood cell membrane and the plasma would contain agglutinin anti-B. The blood group would be A. Blood group A may be homozygous AA and heterozygous Ai. Blood group B may be heterozygous Bi, blood group AB is heterozygous AB (co-dominant). Blood group O is always homozygous ii. The four blood groups do not change during the life time of any human being.

Importance of blood group
Blood grouping is important in:
(a) Transfusion
(b) Establishment of paternity

If a person with blood group A gives blood to a person with type B there will be clumping of blood due to presence of anti A in the blood group B and the recipient will probably die. A person with blood group AB has neither anti A nor anti B plasma antibody and can safely receive A, B or O blood group. A person with blood group O has no cell antigen and can safely give blood to any of the other type. The person with blood group O is therefore known as a universal donor.

Rh Blood Group
The Rh blood group was named after the Rhesus monkey, in which it was first studied by Landsteiner in 1930’s. In humans this group includes antigens or factors called Rh factor. If Rh factor is present in the red blood cell membranes, the blood is said to be Rh positive and if the red blood cells lack Rh factor, the blood is called Rh negative.

Genes of Rh factor
Rh blood group is encoded by three genes C, D, and E. These genes occupy two tightly linked loci. D occupy one locus called locus D. The genes C and E alternatively occupy other loci. The most important of these is D locus.

Gene D
Gene D has two alleles, D and d. Gene D is completely dominant over gene d. Persons having genotype DD or Dd have Rh factor on their red blood cells and are Rh positive. Persons with genotype dd do not have Rh factor and are Rh negative.

Antibody against Rh antigen
This antibody is not present in the ABO blood groups. It is only formed when Rh antigen comes in contact with A/B/AB/O blood groups. So the blood of the Rh negative person contains neither Rh antigen on its cells or Rh antibody in plasma. If the Rh negative person is given Rh positive blood, it will stimulate the plasma to produce antibody. If the same Rh negative person is given Rh positive blood, the antibody already formed in the plasma against Rh positive will react with donated blood. The person can even die. So for the transfusion of the blood, not only blood group A, AB, B, O is matched but the Rh factor is also matched i.e. Rh positive blood can be given only to Rh positive person and Rh negative blood can be given only to Rh negative person. So ABO blood group is written as:

Erythroblastosis Fetalis
The Rh factor is particularly important during pregnancy. Rh positive is dominant over Rh negative. If the mother is Rh negative (dd) and father's genotype is DD, all the offspring (Dd) will be Rh positive.
DD (Father) x dd (mother) = Dd (child)
If the father is heterozygous (Dd) the child has a fifty percent chance of being Rh positive.
Dd (father) x dd (mother) = Dd, Dd, dd, dd (child)
The red blood cells of an Rh+ child will leak across the placental barrier into the mother's circulatory system because placental tissues normally break down before and at birth. The presence of these Rh antigens causes the mother to produce anti- Rh antibodies. In this or a subsequent pregnancy with an Rh-positive baby, anti-Rh antibodies produced by the mother may cross the placenta and destroy this child's red blood cells. This is called hemolytic disease of the now born or erythroblastosis fetalis. This anemia may lead to abortion or still birth. Even if the pregnancy continues, the liver and spleen of the fetus swell as they rapidly produce red blood cells. The breakdown product is called bilirubin. Haemolysis continues after the baby is born. Due to red blood cell destruction followed by haeme breakdown, bilirubin rises in the blood. Excess bilirubin can lead to brain damage, mental retardation, jaundice and even death.
Haemolytic disease of the new born
Prevention: The problem has been solved by giving Rh- negative women an Rh immunoglobin injection either midway through the first pregnancy or no later than 72 hours after giving birth to an Rh-positive child. This injection contains anti-Rh antibodies that attack any of the baby's red blood cells in the mother's blood before these cells stimulate her immune system to produce her own antibodies. This injection is not beneficial if the woman has already begun to produce antibodies, therefore, the timing of the injection is most important.

Test of Rh factor: To test if an individual is Rh negative or Rh positive blood is mixed with an anti Rh antibodies. When Rh-positive blood is mixed with anti-Rh antibodies, agglutination occurs.

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