Bombay Blood Group System and Para Bombay Blood Types

Bombay Blood Group (Oh) is a type of blood group in which the individuals do not have A-antigen, nor B-antigen nor H-Antigen but rather have all the antibodies against the A,B and H antigens. The Bombay phenotype was first discovered in 1952 by Dr. Y. M. Bhende in India in a place called Bombay which is the present day Mumbai. In order to better understand the Bombay blood group, you need to understand how blood grouping is done.

Understanding the Bombay Blood Group relative to other blood groups

When an individual is said to have blood group A, it means there are A-antigens on the red blood cells of the individual but B-Antibodies in the serum (the serum is the fluid part of the blood). So also, when someone has blood group B, it means the person has B-Antigens on the surface membrane of the red blood cells but A-antibodies in the serum. If you have both the A and B antigens, it means you belong to blood group AB and there are neither A nor B antibodies; when both the A and B antigens are absent, then you belong to blood group O and therefore has both A and B antibodies in your serum.

Antigens are molecules that may be bonded to other molecules that are capable of triggering immune responses in the body this means that antigens trigger the body to produce antibodies that destroy antigens that are not of the body. Now, aside the Blood group antigens that help us in classifying our blood into different blood types, there are other antigens located on the surfaces of the red blood cells; one of the antigens of importance in our discussion of Bombay blood group is the H-antigen. The H- antigen is found on the surface of all red cells of the different blood types whether you are blood group A, B, AB or O, you will still have the H-antigen on your red blood cells.

Bombay blood group testing showing how reaction of the antigens and antibodies relative to other blood groups
Bombay blood group testing showing how reaction of the antigens and antibodies relative to other blood groups

 

How does the Bombay Blood Group now differ from other blood types?

Just as I stated above, the individuals with Bombay blood group have neither A nor B antigens this places them in Blood Group O then; but the problem is that they also do not have the H-Antigen. This means if you place them as Blood group O and give them group O blood, they will still have transfusion reaction because they have antibodies against the H-antigen of any group-O blood given to them. Therefore it was concluded that this type of blood group cannot be group O and was then named as Bombay Blood Phenotype after the city in India where it was first discovered. And the symbol for representing the Bombay blood type is a capital O with a subscript of small letter h because it seems to be a blood group-O without H-antigen hence Bombay blood group is represented as: Oor hh or H null

The difference between the Bombay blood group and blood group O therefore is the fact that Blood group-O individuals have H-antigens on the RBCs while that of Bombay does not have H-antigens on both the RBCs and secretions.

Could the Bombay Blood group be Rhesus Positive or negative?

Yes the Bombay blood group could either be Rhesus positive or Rhesus negative unlike the Rh null blood type that is the rarest in the world. The Bombay blood phenotype is one of the rarest blood groups in India and world at large.

Genetic Variability of Bombay Phenotype and Para Bombay Phenotype

The H-antigen on RBCs is encoded by the H-gene (also called the FUT1 gene), while the H-antigen found in body secretions is encoded by the Se gene(also called the FUT2 gene) – these two genes are both located on chromosome 19 and are involved encoding for an enzyme called fucosyltransferase that adds fucose to a precursor oligosaccharide. When Homozygous alleles for defective Se (sese) gene are formed, this results in the non-secretor phenotype with an incidence of approximately 20%. The rare Bombay phenotype occurs as a result homozygous inactive alleles at H and Se this causes this type of Bombay individuals to lack the H-antigen on both RBCs and secreted proteins and therefore make potent antibodies against the H antigen.

Para Bombay Blood Group

For Para-Bombay Phenotype, there are two different situations that can occur: the first situation occurs when individuals have homozygous nonfunctional allele at FUT1 (gene for producing H antigen on RBCs) but also have at least one functional Se allele (gene for producing H antigen in secretions) this makes them lack H antigen on their RBCs but will have the H antigen in their secretions; the second situation occurs when individuals have minimal residual H production on their RBCs and are non-secretors. What this means is that the Para Bombay antibodies are not as potent as the Bombay antibodies produced against the H-antigens because the Para Bombay phenotype has some limited production of the H-antigens either on RBCs or in secretions.

How Blood groups are formed at the molecular level

The H substance is bio-chemically produced by the binding of Fucose to the surface glycoproteins by the help of the enzyme known as fucosyltransferase. Blood group A is formed when N-acetyl-galactosamine binds to the H substance. If galactose binds to the H substance, blood group B is formed. If neither substance binds to the H substance, then blood group-O is formed. Individuals that fail to express H transferase (FUT1) lack the H antigen, which is the foundation of Bombay phenotype. Individuals that lack this expression cannot synthesize A or B antigenic structures regardless of their ABO blood given genotype, and A,B, and H antigens are absent from both their erythrocytes and secretions.

Bombay Blood Group Testing

During normal cell grouping or forward grouping , Bombay blood group would be categorized as O group because it would not show any reaction to anti-A and anti-B antibodies just like a normal O group would do but when cross matched with different blood bags of O group, then it would show cross-reactivity or incompatibility which calls for further test such as reverse grouping or serum grouping to be performed.

Clinical Significance of H-antibodies in Bombay Phenotypic individuals

  1. Transfusion reactions: when people with antibodies against H antigens receive transfusions of blood that contains the H antigen such as from blood group O individuals, they are at risk of suffering an acute hemolytic transfusion reaction.
  2. Hemolytic disease of the new born: during pregnancy, the maternal production of anti-H antibodies could cause hemolytic disease in a fetus who did not inherit the mother’s Bombay phenotype; but in actual practice, cases of HDN caused in this way have not been described, possibly because of the rarity of the Bombay phenotype.