Certain chemical compounds in red blood cells determine the blood type depending upon the genes you inherit from your parents. The blood group does not change during the lifetime.
There are four main blood groups: A, B, AB and O. If you belong to group A, this means that you have in your red blood cells, a chemical substance called A, a group B person has the B substance, AB has both A and B substances, while group O has neither of them.
These groups substances can be identified by their reaction with two antibodies (Anti-A and Anti-B) which exist in the plasma. A characteristic of these antibodies is that they will cause red cells containing a particular group substance to come together in clumps and if strong enough, may, destroy the cells completely. Thus Anti –A will ‘clump’ cells containing A (Groups A and AB), while Anti-B will clump cells containing B (Groups B and AB). Neither of them will react with group O cells. You will not usually have in your plasma an antibody which would react with the group substance in your own red cells.
When a transfusion is needed, the blood given to a person must match his / her type. The red cells of the blood selected must belong to a group, which will not be affected by any antibody in the patient’s plasma. The compatibility test between the donor and the recipient known as cross matching is done to prevent any transfusion reaction and ensure maximum benefit to the patient.
Because O group blood may be used in emergency situations for A, B or AB group recipients it has been known as Universal Donor.
The incidence of A B O groups in India varies in different parts. In western countries A group is common while India B group frequency is more.
A certain substance known as ‘Rh factor’ is also either present or absent on the red cells. 95% of Indians have it (Rh positive) and the rest don’t (Rh negative). Thus your blood group could be either of the following 8 groups:
A Rh Positive A Rh negative
B Rh Positive B Rh negative
AB Rh Positive AB Rh negative
O Rh Positive O Rh negative
If Rh Positive blood where given to an Rh negative patient it is likely that an antibody to Rh factor would be formed. This is known as sensitization. The antibody would stay in the blood stream and cause trouble if Rh Positive were to be transfused subsequently.
A body born to an Rh negative mother married to Rh positive father may be Rh positve or Rh negative. At the time of delivery blood leaks from the infant into the circulation of the mother and she becomes permanently sensitized. The first Rh positive baby is rarely affected because the mother usually does not become sensitized to the Rh factor until the child is born.
If the mother becomes pregnant again with a Rh positive baby, she may respond by making an Rh antibody which passes into the baby and attacks its red cells leading to ‘hemolytic disease of the newborn’. Eventually the baby is born with anemia and jaundice. Such a baby is known as a Rhesus baby. The only way of saving the baby’s life is a complete change of blood by gradual removal of the baby’s blood and replacement by fresh blood. This process is known as exchange transfusion.
It is possible to prevent an Rh-negative mother being sensitized by administration of an injection of Rh immunoglobulin with in 72 hours of birth of the Rh-positive baby. It is of no value to already sensitized mothers. It is therefore necessary for Rh-negative mothers to have a periodical blood check up during pregnancy.
