Bombay Blood Group (hh Phenotype) — The Rarest Blood Type

The Bombay blood group (hh phenotype / Oh group):

• First discovered: 1952, in Bombay (Mumbai), India • Discoverer: Dr. Y.M. Bhende (Indian haematologist) • Genotype: hh (homozygous recessive for the H gene) • H antigen: ABSENT from red blood cells (unlike all other ABO groups)

Normal ABO blood groups: • All ABO groups (A, B, AB, O) carry the H antigen • The H antigen is the base structure onto which A or B antigens are added • Group O: has H antigen but neither A nor B antigens

Bombay blood group: • Does NOT have the H antigen • Cannot form A or B antigens • Has anti-H antibodies in plasma (along with anti-A and anti-B) • On routine ABO typing, appears as blood group O — but is fundamentally different

Key distinction: Bombay group ≠ Blood group O Blood group O has H antigen (no A or B) Bombay group has NO H antigen at all

Genetic basis: • The H antigen is coded by the H gene (FUT1 gene) on chromosome 19 • Normal allele: H (dominant) — produces H antigen • Bombay allele: h (recessive) — cannot produce H antigen

Genotypes: • HH or Hh: H antigen present → normal ABO group • hh: No H antigen → Bombay blood group

Since hh is homozygous recessive, both parents must carry at least one h allele for a child to have the Bombay phenotype.

Frequency: • India (Mumbai/Bombay region): 1 in 10,000 people • Western India: relatively more common • Europe/rest of world: 1 in 1,000,000 people • Overall: extremely rare globally

The higher frequency in Mumbai may be due to historical founder effect and consanguineous (related) marriages in certain communities.

Critical medical implications:

1. Transfusion compatibility: • People with Bombay blood group can ONLY receive blood from other Bombay blood group donors • Cannot receive blood of any ABO group — even O negative — because all other blood has H antigen • Their plasma contains anti-H antibodies that attack H antigen on all other blood types → severe transfusion reaction (hemolysis)

2. Blood transfusion crisis: • In medical emergencies, finding a compatible donor is extremely difficult • Bombay blood group individuals must pre-donate and store their own blood (autologous donation) • Blood banks in India (especially Mumbai) maintain a registry of Bombay blood group donors

3. H antigen antibodies: • Anti-H antibodies in Bombay group are IgM class — very potent • Can cause life-threatening hemolytic transfusion reactions

4. Inheritance: • Children of Bombay group parents may or may not have the phenotype depending on H gene status • If both parents are hh, all children will be Bombay group

Blood group registries: • Rare Donor Programme (ISBT) maintains a global list • In India, hospitals in Mumbai, Chennai, and Delhi maintain Bombay group donor registries