The Bombay Phenotype is a rare blood type that mimics Type O but can only receive blood from its own kind.

The Bombay Phenotype, also known as the hh blood group or Oh, was first identified in 1952 by Dr. Y.M. Bhende at the Seth Gordhandas Sunderdas Medical College in Mumbai. In standard ABO blood grouping, the H antigen serves as the essential precursor molecule. Enzymes normally convert this H antigen into A or B antigens depending on a person's genetics. Individuals with the Bombay Phenotype lack the FUT1 gene required to produce the H carbohydrate chain.Because the H antigen is the foundation of all ABO blood types, its absence means the person cannot express A or B antigens even if they possess those specific genes. On a standard blood test, the lack of A and B antigens causes the blood to be categorized as Type O. However, the immune system of a Bombay Phenotype individual recognizes the H antigen as a foreign substance. This leads to the production of potent anti-H antibodies that are not present in any other blood group.This immunological profile creates a medical emergency during transfusions. While Type O is usually the universal donor, it contains the H antigen, meaning a Bombay Phenotype patient's body will violently reject it. This condition is exceptionally rare, appearing in roughly 1 out of every 1,000,000 people in Europe and 1 in 250,000 people globally. However, due to founder effects and consanguinity, the rate is significantly higher in parts of India, reaching 1 in 10,000 residents in the Mumbai region.