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What is a secret blood type?

There are many different blood types that most people are familiar with, such as A, B, AB and O. However, there are some lesser known and rare blood types that are considered ‘secret’ due to their obscurity. These secret blood types include things like hh, Bombay phenotype, PP-null, and golden blood. While these blood types are exceedingly rare, understanding their unique properties and characteristics provides insight into the complexities of human blood.

What Determines Blood Type?

Blood type is determined by the presence or absence of inherited antigenic substances on the surface of red blood cells. The main antigens that determine blood group are:

  • Antigen A
  • Antigen B
  • Rh antigen (positive or negative)

The distribution of these antigens gives rise to the common ABO blood type groupings:

Blood Type Antigens Present
AB A and B
O Neither A nor B

The presence or absence of Rh factor gives the positive or negative designation. But there are many other rarer antigens that can affect blood type as well. It is some of these lesser known antigens that give rise to the secret blood types.

The Rarest Blood Type: hh

One of the rarest and most secret blood types is the Bombay phenotype or hh blood. This blood type was first discovered in Bombay, India in 1952. The hh blood group contains no A, B, or Rh(D) antigens on red cell surfaces.

This occurs due to an inherited condition called the Hh blood group. The H antigen is a precursor substance necessary to produce the A, B, and Rh antigens. When the H antigen is missing, none of the usual blood group antigens can be synthesized.

This results in blood that is missing the major antigens present in most people. People with Bombay phenotype can donate red blood cells to any ABO type, making them “universal donors” similar to O negative blood type. However, they can only receive blood from other hh donors, which are incredibly scarce.

It is estimated that only 1 out of 250,000 people have Bombay phenotype blood. Certain populations such as Reunion Island in the Indian Ocean have higher frequencies, around 1 out of 7000. But overall this blood type is exceptionally rare worldwide.

The Golden Blood: Rh-null

Rh-null blood is one of the rarest subtypes in the world. It has been described as “golden blood” due to its precious value. This blood type contains no Rh antigens at all.

Most people have Rh positive blood and carry the Rh D antigen. Rh negative blood lacks this particular antigen. But Rh-null blood lacks all 61 possible antigens in the Rh system. This extremely rare condition arises when the RHD gene responsible for Rh antigen production is completely deleted or altered.

Rh-null is the rarest blood group, with only around 50 individuals known worldwide as of 2007. Their blood is considered a “universal plasma donor” since it lacks all Rh antigens. However, Rh-null red blood cells can only be given to others with Rh null blood.

Finding compatible donors has been nearly impossible for many Rh-null individuals. Only 9 active donors worldwide were known in 2007. Luckily, some Rh-null people can safely receive blood from Rh-negative relatives who may share rare partial Rh antigen patterns.

PP-null: The Rarest Blood in P Blood Group System

Most people have the P1 antigen on their red blood cells, giving rise to Pp and PP blood types within the P system. But PP-null blood lacks this antigen entirely due to a mutation in the P1 synthase gene (A4GALT). This mutation prevents conversion of precursor LKE antigen into P1 antigen.

The resulting deficiency of P1 antigen leads to production of anti-P1 antibodies. When PP-null blood is exposed to normal P antigen positive blood, the anti-P1 antibodies attack and agglutinate the red blood cells.

Only 43 PP-null individuals have been reported worldwide as of 2011. Most came from Japanese populations. PP-null blood can safely be given to other PP-null patients. But providing donor blood for PP-null patients is an immense challenge due to the rarity of compatible blood.

Bombay Phenotype Donation Challenges

The extreme rarity of the hh blood type poses significant difficulties for hh blood donors and patients. There are so few hh donors that most individuals cannot get regular blood transfusions. They rely heavily on family hh donors if available. Some hh patients resort to autologous blood donation, where they store their own blood for future use.

Special rare donor registries have been created to try locating hh donors across countries. The International Rare Donor Panel has helped coordinate sourcing of hh blood between nations when local donors cannot be found.

Once an hh donor is identified, donation itself also poses challenges. Most blood centers are not properly equipped to draw, test, and store this rare blood type. Large quantities of blood may be drawn each time for freezing, as future compatible donors are unlikely. The short shelf life of red blood cells also means this precious frozen supply must be continually replenished.

The Future of Rare Blood Management

Advancements in rare blood typing and donor screening are still needed to improve diagnosis and sourcing of unusual blood types like hh and Rh-null. Increased awareness and efficient sharing of rare donor registries globally can help connect patients with lifesaving donors more quickly.

Developing standardized guidelines for rare donor selection, blood collection, processing, and distribution will also streamline efforts between countries. Centralization of rare blood into networked storage facilities may help maximize access. Rare donor organizations suggest concentrating frozen rare blood reserves in large centralized repositories rather than smaller scattered ones.

Scarce red blood cells will likely remain an ongoing challenge. But developing synthetic blood alternatives may someday ease reliance on rare donors. Current experimental approaches focus on generating cultured red blood cells in the lab for transfusion. Progress has been made using stem cells, immortalized cell lines, and other innovative techniques. These cutting edge technologies may one day help generate customized blood on demand.


While the common blood types of A, B, AB and O are familiar to most, the world of rare blood is filled with exotic and secrets antigens and groups. Bombay phenotype, Rh-null, PP-null and other similar blood types arise from unusual genetic mutations that remove major red blood cell antigens. The absence of these ubiquitous antigens makes such blood exceptionally rare worldwide.

Sourcing compatible blood for those lacking common antigens poses an immense challenge for rare blood donors and patients. But global cooperation between blood banks is helping locate donors and distribute these precious pints when no other option remains. Ongoing advances and awareness of rare blood types will hopefully improve identification, typing, and access to protect the lives of these unique patients. While exotic blood types like hh and Rh-null remain obscure for now, their study provides insight into the intricacies of transfusion medicine and genetics.