ABO blood group system:
The human population is divided into four groups according to the ABO blood group system, first described by the Austrian pathologist, Karl Landsteiner. They are A type, B type, AB type and O type blood groups The capital letters A and B represent A-antigen and B-antigen respectively present in the individual's red blood cells .The corresponding antibodies are represented by small letters a and b, and are present in the blood plasma .An individual having a particular antigen on his red blood cells will not have the same antibody in his plasma, as it would cause agglutination .The immune system is tolerant to its own red blood cell antigens. So a person with blood group A does not produce antibody a but has antibody b, a person with blood group B will have antibody a and not antibody b, a person with blood group AB will have both the antigens A and B but will not have the antibodies a and b, a person with blood group O will not have both the antigens A and B but will have the antibodies a and b.
A simple compatibility test is done to determine to which group a sample of blood belongs. Generally blood transfusions are carried out using donor blood that belongs to the same group as the recipient. Persons of blood group A can receive blood from A type and O type, persons with B type blood group can receive blood from B type and O type, persons with AB type can receive blood from all types -A, B, AB and O type and persons with O type blood group can receive blood from O type only.
Individuals of blood group O are referred to as universal donors as they can donate blood to a recipient of any blood group without causing agglutination. Individuals of blood group AB are called universal recipients as they can receive blood from donor of any blood group without causing agglutination.
Figure 1 clearly depicts the compatibility of different blood groups .The capital letters indicate the antigens on the Red Blood Cells and the small letters the antibodies in the plasma. - denotes no agglutination and + denotes agglutination.
Figure 1
Multiple alleles (alternate states of a gene) control the ABO blood group. The human gene I determines the ABO blood type and exhibits more than one codominant allele .The three I gene alleles are IA, IB and i. Both the alleles IA and IB are dominant over allele i. An allele that is expressed in the phenotype is called a dominant allele. The alleles IA and IB are codominant.
Each individual has two copies of the chromosome bearing I gene. Different combinations of the three I gene allele result in four different blood type phenotypes.
- Individuals with blood type A may be either IA IA or IA i.
- Individuals with blood type B may be either IB IB or IB i.
- Individuals with blood type AB are IA IB. Individuals with alleles IA IB produce both the antigens A and B. As both the alleles are expressed simultaneously, IA and IB alleles are referred to as codominant.
- Individuals with blood type O are ii.
Figure 2 depicts the different combination of the I gene allele resulting in four different blood type phenotypes.
Figure 2
Rhesus blood group system:
The Rhesus factor is so called as it was first recognized in Rhesus monkeys. Individuals with the Rhesus factor are called Rhesus positive and individuals without the Rhesus factor are called Rhesus negative. People with Rhesus negative factor do not inherently have Rhesus factor antibodies. But if Rhesus positive blood is introduced into Rhesus negative recipient, the individual responds by producing the corresponding Rhesus antibodies. Subsequently, if the same Rhesus negative person again receives Rhesus positive blood, the Rhesus antibodies will cause agglutination of the Red Blood Cells of the donor's blood, which often leads to death.
Blood group incompatibility between mother and child.
Incompatibility between the blood groups of the mother and the child (foetus) may lead to haemolytic disorder in the child as seen in the case of Rhesus factor incompatibility.
Rhesus factor incompatibility.
Rhesus factor is expressed by dominant gene R. Rhesus positive individuals are, therefore RR or Rr and Rhesus negative individuals are rr. If both the parents are Rhesus negative, then there is no problem as the baby is also Rhesus negative. But if the mother is Rhesus negative (rr) and the father is Rhesus positive (Rr/RR) then the child may be Rhesus positive (Rr). The problem arises when a Rhesus negative mother bears a Rhesus positive child .Red Blood Cells from the foetus may enter the mother's blood during delivery .The mother's body will recognize the Rhesus (D) antigen as foreign and produce antibodies against the Rhesus factor.
In her subsequent pregnancy, with a Rhesus positive foetus, the Rhesus antibodies from the mother's blood will cross the placenta and enter the foetal circulation and may cause massive destruction of the baby's red blood cells - a condition called haemolytic disease of the new born (erythroblastosis foetalis). This could lead to the death of the baby if the child's blood is not replaced with Rhesus negative blood .A blood transfusion can now be given while the baby is still in the womb. These days, the haemolytic disease can be prevented by injecting the mother with anti- Rhesus antibodies called Anti-D that attaches themselves to the Rhesus antigens of the foetal cells, that are in the mother's blood and thus prevent them from being recognized by the mother's antibody forming cells.