Blood can be transfused from one individual to the other in case of emergency. Blood transfusions were tried as early as 18th century in England and France. But some cases were successful and in some instances the recipient died soon after transfusion. On testing it was noticed that the blood had coagulated or clumped.
However this did not happen in all instances. In some instances the blood of two individuals mixed smoothly. It was Karl Landsteiner who in 1900 found the reason behind clumping or smooth mixing. He noticed that the plasma and red blood cells (RBC) of a person when recombined after separation mixed smoothly so also blood from some persons.
But in certain other instances the plasma and the RBC did not mix, the RBC clumped. Landsteiner worked in detail on this and came to the conclusion that the mixing or clumping depends on the antigen and antibody reaction present in the blood. He discovered that there are two antigens in the blood. A and B and two serum antibodies that clump or agglutinate them. Correspondingly the blood of all human beings could be classified into four types based on the antigen present in the RBC. These are
1. Antigen A- Blood group A
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2. Antigen B- Blood group B
3. Both Antigen A and B- Blood group AB
4. No Antigen- Blood group O
Before we proceed on further discussion we will try to understand what is meant by antigen – antibody reaction.
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[ Blood is a connective tissue in fluid form. It has two main parts – cells and plasma. The cells are RBC, WBC and platelets. The plasma consists of protein, fibrinogen and the serum.
When the serum is injected into the blood stream of another organism, a substance is produced in the blood which reacts with the serum protein and neutralizes it. The substance produced by the blood cell is known as antibody and the protein in the serum which causes the production of antibody is called antigen. Antibody is also called Agglutinin (because it agglutinates) and antigen is very specific and is often compared to the lock and key mechanism.]
Landsteiner discovered A,B and O blood groups. His students Von Decastello and Sturii discovered the AB groups. Besides these, scientific investigations have also proved the existence of two other blood groups viz MN blood groups and Rh factors. MN blood groups are typical Mendelian characters. They are controlled by a single gene.
A, B, AB, and O groups: In these blood groups, some of them contian natural antibodies against certain others. It has been pointed out already, there are two antigens (agglutinogen) A and B and there are two antibodies (agglitinin) A and B. Persons of blood group A contain within the serum of their blood an antibody which agglutinates RBC of blood group B. Group B serum has antibody against A.
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The AB group serum however has no antibodies while O group has both antibodies for A and B. Hence O group cannot receive blood from any group except itself. But it has no antigens and can be given to any other group (it does not cause the production of antibodies in blood types, the following table gives the antibody and antigens of various blood groups.