When blood from two individuals is mixed, sometimes clumping (agglutination) of RBCs occurs. This clumping is because of the immunological reactions. But, why clumping occurs in some cases and not in other cases remained a mystery until the discovery of blood groups by the Austrian Scientist Karl Landsteiner, in 1901. He was honored with Nobel Prize in 1930 for this discovery.


Determination of ABO blood groups depends upon the immunological reaction between antigen and antibody. Landsteiner found two antigens on the surface of RBCs and named them as A antigen and B antigen. These antigens are also called agglutinogens because of their capacity to cause agglutination of RBCs. He noticed the corresponding antibodies or agglutinins in the plasma and named them anti-A or α-antibody and anti-B or β-antibody. However, a particular agglutinogen and the corresponding agglutinin cannot be present together. If present, it causes clumping of the blood. Based on this, Karl Landsteiner classified the blood groups. Later it became the ‘Landsteiner Law’ for grouping the blood.


Landsteiner law states that:

1. If a particular agglutinogen (antigen) is present in the RBCs, corresponding agglutinin (antibody) must

be absent in the serum.

2. If a particular agglutinogen is absent in the RBCs, the corresponding agglutinin must be present in the

serum. Though the second part of Landsteiner law is a fact, it is not applicable to Rh factor. BLOOD GROUP SYSTEMS

More than 20 genetically determined blood group systems are known today. But, Landsteiner discovered

two blood group systems called the ABO system and the Rh system. These two blood group systems are the most important ones that are determined before blood transfusions.


Based on the presence or absence of antigen A and antigen B, blood is divided into four groups:

1. ‘A’ group

2. ‘B’ group

3. ‘AB’ group

4. ‘O’ group.

Blood having antigen A belongs to ‘A’ group. This blood has β-antibody in the serum. Blood with antigen B and α-antibody belongs to ‘B’ group. If both the antigens are present, blood group is called ‘AB’ group and serum of this group does not contain any antibody. If both antigens are absent, the blood group is called ‘O’ group and both α and β antibodies are present in the serum. Antigens and antibodies present in different groups of ABO system. Percentage of people among Asian and European population belonging

to different blood group. ‘A’ group has two subgroups namely ‘A1’ and ‘A2’. Similarly ‘AB’ group has two subgroups namely ‘A1B’ and ‘A2B’.


Determination of the ABO group is also called blood grouping, blood typing or blood matching.

Principle of Blood Typing – Agglutination

Blood typing is done on the basis of agglutination. Agglutination means the collection of separate particles

like RBCs into clumps or masses. Agglutination occurs if an antigen is mixed with its corresponding antibody which is called isoagglutinin. Agglutination occurs when A antigen is mixed with anti-A or when B antigen is mixed with anti-B.

Requisites for Blood Typing

To determine the blood group of a person, a suspension of his RBC and testing antisera are required. Suspension of RBC is prepared by mixing blood drops with isotonic saline (0.9%).

Test sera are:

1. Antiserum A, containing anti-A or α-antibody.

2. Antiserum B, containing anti-B or β-antibody.


1. One drop of antiserum A is placed on one end of a glass slide (or a tile) and one drop of antiserum B on the other end.

2. One drop of RBC suspension is mixed with each antiserum. The slide is slightly rocked for 2 minutes.

The presence or absence of agglutination is observed by naked eyes and if necessary, it is confirmed by

using microscope.

3. Presence of agglutination is confirmed by the presence of thick masses (clumping) of RBCs

4. Absence of agglutination is confirmed by clear mixture with dispersed RBCs.


1. If agglutination occurs with antiserum A: The antiserum

A contains α-antibody. The agglutination occurs if the RBC contains A antigen. So, the blood

group is A.

2. If agglutination occurs with antiserum B: The antiserum B contains β-antibody. The agglutination

occurs if the RBC contains B antigen. So, the blood group is B.

3. If agglutination occurs with both antisera A and B: The RBC contains both A and B antigens to cause

agglutination. And, the blood group is AB.

4. If agglutination does not occur either with antiserum A or antiserum B: The agglutination does not occur

because RBC does not contain any antigen. The blood group is O.

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