ABO grouping is based on the presence or absence of two surface antigen on the RBCs namely A and B.
The plasma of different individuals contains two natural antibodies.
The distribution of antigens and antibodies in the four groups of blood, A, B, AB and O.
The blood of a donor has to be carefully matched with the blood of a recipient before any blood transfusion to avoid severe problems of clumping, which leads to destruction of RBC.
Group ‘O’ blood can be donated to persons with any other blood group and hence ‘O’ group individuals are called ‘universal donors’.
Persons with ‘AB’ group can accept blood from persons with AB as well as the other groups of blood, and such persons are called ‘universal recipients’.
The Rh antigen similar to one present in Rhesus monkeys is also observed on the surface of RBCs of majority of humans, hence the antigen is known as Rh antigen.
The individuals having Rh antigen are called Rh positive (Rh+ve) and those in whom this antigen is absent are called Rh negative (Rh-ve).
An Rh-ve person, if exposed to Rh+ve blood, will form specific antibodies against the Rh antigens, and hence Rh group should also be matched before transfusions.
A special case of Rh incompatibility has been observed between the Rh-ve blood of a pregnant mother with Rh+ve blood of the foetus , which leads to a disease known as erythroblastosis foetalis.
Rh antigens of the foetus do not get exposed to the Rh-ve blood of the mother in the first pregnancy as the two bloods are well separated by the placenta, during the delivery of the first child, maternal blood may get exposed to small amounts of the Rh+ve blood from the foetus and the mother starts preparing antibodies against Rh in her blood.
In case of subsequent pregnancies, the Rh antibodies from the mother (Rh-ve) can leak into the blood of the foetus (Rh+ve) and destroy the foetal RBCs, which cause severe anaemia and jaundice to the baby leading to a condition known erythroblastosis foetalis.
Erythroblastosis foetalis can be avoided by administering anti-Rh antibodies to the mother immediately after the delivery of the first child.