Blood groups system was discovered at the beginning of the twentieth century by Karl Landsteiner. The blood groups were divided in A, B, AB and O. Classification and nomenclature of different blood group is based on the presence or absence of inherited antigenic substances on the surface of red blood cells (RBCs).

According to karl landsteiner law-

  • If an agglutinogen (antigen) is present on the surface of red blood cells of an individual, the corresponding agglutinin (antibody) must be absent in the plasma. 
  • If an agglutinogen (antigen) is absent on the surface of red blood cells of an individual, the corresponding agglutinin (antibody) must be present in the plasma.

Inheritance of ABO blood group:

Bernstein first described the theory of inheritance of ABO blood groups. He demonstrated that each individual inherits one ABO gene from each parent and the presence of these two genes determines the type of antigen present on the surface of red cells. The gene A, B or O occupy one locus. The ABO gene locus is located on the chromosome 9. A child receives one ABO gene from each parent, giving rise to six possible genotypes and four possible blood groups (phenotypes).

Antigen of ABO group:

Antigens are the complex structures that contain protein and carbohydrate. Blood group antigens (A antigen and B antigen) is present on the surface of red blood cells. Biochemical analysis of blood group antigens has shown that they fall into two main types:

Protein determinants:- It represent the primary products of blood group genes.

Carbohydrate determinants:- In this type, the products of the genes controlling antigen expression are glycosyltransferase enzymes.

Some antigens are defined by the amino acid sequence of a glycoprotein, but are dependent on the presence of carbohydrate for their recognition serologically.

  • A and B genes do not produce antigens directly.
  • Synthesis of blood group antigens requires at least two steps. First is synthesis of H antigen and the second is synthesis of either A or B structure or both AB structure.  
  • RBC have oligosaccharide molecule on its surface consists of glucose, galactose, N-acetylglucosamine and galactose (RBC precursor substances).
  • The H gene codes for an enzyme (fucosyltransferase) that adds the sugar fucose to the terminal sugar of a precursor substance, which forms H antigen. Maximum amount of H antigen is found on O blood group.
  • In the formation of A and B antigens, H antigen acts as a precursor.
  • The “A” gene codes for an enzyme (N-acetylgalactosaminyl transferase) that adds N-acetylgalactosamine to the terminal sugar of the H antigen, which form A antigen (A blood group).
  • The “B” gene codes for an enzyme (D-galactosyl transferase) that adds D-galactose to the terminal sugar of the H antigen, which form B antigen (B blood group).
  • If both of them are added to the terminal sugar of the H antigen, it forms A antigen and B antigen (AB blood group).

The H antigen is present on the red cells in the following diminishing quantity.

O > A2 > B > A2B > A1 > A1B

The ABO antigens are found on all the cells of the body tissues. The ABO compatibility is a prerequisite in cases of organ transplants.

Antibody of ABO group:

ABO antibodies are generally IgM in nature, which are not able to pass through the placenta to the fetal blood circulation. They are naturally occurring as they do not need any stimulus. Antibodies are formed as a result of transformation. If an agglutinogen (antigen) is absent on the surface of red blood cells of an individual, the corresponding agglutinin (antibody) is present in the plasma. Person with O blood group have more anti-A and anti-B than any others.

The IgM anti-A and anti-B are not present in newborn, up to the age 3 to 6 months. The maximum titre reaches by the age of 5 to 10 years.

NOTE:-

“O” blood group has no antigen and agglutination does not occur with any other blood group. So, “O” blood group can be given to the person with other blood group. “O” blood group is called “universal donors”.

Plasma of AB blood group has no antibody. This does not cause agglutination of RBC from any other blood group. People with AB blood group can receive from any blood group person. So, this blood group is called “universal recipients”.

Subgroups of A:

Blood group ‘A’ has two subgroups- A1 and A2. A1 subgroup reacts with both anti-A and anti- A1. Whereas A2 subgroup only reacts with anti-A and not reacts with anti- A1.

80% of A or AB blood group individuals are subgroup A1 and A1B, but only 20% of A or AB blood group individuals are subgroup A2 and A2B.

Subgroups of B:

B subgroup occurs less than A subgroup. The variants of ‘B’ can be classified into B3, Bx, Bm and Bel. They are differentiated by the type of reactions with anti-B, anti-A, B anti-H. Occurrence of B subgroup is very rare in general population.

Subgroups of AB:

Like A group, the AB group is also sub classified in A1B and A2B subgroups.

Bombay blood group (Oh phenotype)

Bombay blood group is a rare blood group which was first discovered in Bombay, now called Mumbai, in India. Only 4 per million of the world population have this blood group.

Blood group ‘O’ do not carry either A or B antigen, but have maximum amount of H antigen on their red blood cells. Some individuals lack even H antigen along with A and B. These individuals are called Oh phenotype. Since there is no H antigen on the surface of red cells of Oh, the anti-H antibody develops in their serum, along with all the other antibodies found in any O blood group. The genotype of Bombay blood group is h/h.

Bhende YM, et al in the year 1952, first discovered this blood group in the city of Bombay, India, from where it got its name. The Bombay blood group is not compatible with any ABO blood group, so they can donate their red cells to any other member of the ABO blood group system, but they cannot receive blood from any other member of the ABO blood group system.