Hh blood group

Rare blood group lacking H antigen

The Hh blood group, also known as the Bombay blood group, is a rare blood group phenotype characterized by the absence of the H antigen on red blood cells. This unique blood group was first discovered in Bombay, now Mumbai, India, in 1952 by Dr. Y. M. Bhende. Individuals with this blood group are unable to produce the H antigen, which is a precursor to the A and B antigens found in the more common ABO blood group system.

Genetics

The Hh blood group is inherited in an autosomal recessive manner. The gene responsible for the production of the H antigen is the FUT1 gene, located on chromosome 19. Individuals with the Bombay phenotype have two non-functional alleles of the FUT1 gene, resulting in the absence of the H antigen on their red blood cells. Consequently, they cannot express A or B antigens, regardless of their ABO genotype.

Biochemistry

Bombay blood group distribution

The H antigen is a carbohydrate structure that serves as a foundation for the A and B antigens. In individuals with the Bombay phenotype, the lack of functional FUT1 enzyme prevents the conversion of precursor substances into the H antigen. As a result, these individuals have anti-H antibodies in their plasma, which can cause severe hemolytic transfusion reactions if they receive blood containing the H antigen.

Clinical significance

The presence of anti-H antibodies in individuals with the Bombay phenotype makes blood transfusions particularly challenging. These individuals can only receive blood from other Bombay phenotype donors, as even O group blood, which is typically considered a universal donor, contains the H antigen. The rarity of the Bombay blood group means that finding compatible donors can be difficult, especially outside of regions where the phenotype is more prevalent.

Prevalence

The Bombay blood group is extremely rare globally, with a higher incidence in certain populations, such as in parts of India. The frequency of the Bombay phenotype is estimated to be about 1 in 10,000 in India, but it is much rarer in other parts of the world.

Related pages

• ABO blood group system
• Rh blood group system
• Blood transfusion
• Hemolytic transfusion reaction