The under­stood until about 1900. Thereafter, it

The practice of transfusion was tried as early as 18th century in France and later in England. Some cases of transfusion were successful but in others fatal transfusion reactions resulted.

This led the people to think that why did some people die after a transfusion, while others did not show any such reaction, although the same sterile technique of administering the blood was followed in all cases.

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It could not have been infection; therefore, there must be something in the blood which made the two bloods incompatible. In what way they were incompatible; what actually happened?

The reason for adverse result of transfusion was not under­stood until about 1900.

Thereafter, it was found that in the transfu­sion reaction the red blood cells clumped together or agglutinated and then underwent dissolution or haemolysis.

This so affected the kid­neys that they could no longer perform their function of removing the waste materials of the body and the patient dies of toximia.

The agglutination of red blood cells is due to protein substances in the red blood corpuscles coming in contact with incompatible substances in the blood plasma. It is an antigen-antibody reaction.

The substances which found in red blood cells are called antigens or agglutinogens where as which are found in blood plasma called antibodies or agglutinins.

Antigens are proteins capable enough to stimulate the production of specific antibodies. Antibodies on the other hand, are the substances produced in the body in response to counteract with foreign antigens and react specifically to particular antigen.

In the human blood two types of antigens namely A and B are found in the red blood cells and two types of antibodies namely a (anti A) and b (anti B) are found in the blood plasma.

The blood antigens are glycoproteins while the antibodies are specific proteins in the plasma and are called natural antibodies.

Agglutination of red blood cells occurs when antigen A comes in contact with agglutinin or antibody a and when antigen B comes in contact with agglutinin or antibody b.

Based on the mutual compatibility of these antigens and anti­bodies Karl Landasteiner dicoveredfour blood groups in the human blood in the year of 1900. They are:

Type A has the antigen A and antibody b

Type B has the antigen B and antibody a

Type AB has both A and B antigens but no antibody.

Type O has both a and b antibodies but no antigen.

These above blood groups show that only these antibodies or agglutinins can remain in the blood whose corresponding antigen or agglutinogen is not present.

It will be evident by looking at the above blood groups that blood of group A can be given only to those person who do not possess antibody a, it means only in A and in AB. Similarly the blood of group B can be given only to B and AB.

AB blood can be given to only AB recipient, because in all other types there is always some agglutinin.

O type can be given to O and to any other type because it has no antigen at all. A person with O blood is called a universal donor, while AB is called the universal recipient.