What Is the Rarest Blood Type?

Rh null is an extremely rare type

Medically reviewed by Steffini Stalos, DOMedically reviewed by Steffini Stalos, DO

The rarest blood type in the world is called Rh null. It is sometimes referred to as the golden blood type. Fewer than 100 individuals worldwide are believed to have Rh null blood type, a condition described as Rh deficiency syndrome.

Blood types vary in prevalence, with some being more common and others relatively rare. An individual’s blood type is determined by antigens (substances that can stimulate an immune response) present on the surface of the red blood cells (RBCs).

The major blood types include the ABO system, with types A, B, AB, or O, and the Rhesus (Rh) system, which denotes Rh positive (+) or Rh negative (-) for the presence or absence of the Rh(D) antigen, also called Rh factor. People have both an ABO type and an Rh type, making eight typical combinations.

Only about 15% of the population is Rh negative. Most people, even those who are Rh negative, have other Rh antigens on their RBCs, but people who have Rh null blood type do not have any Rh antigens on their RBCs.

This article will discuss the implications of having a rare blood type, how blood types vary by population, how they are inherited, and compatibility for transfusion.

Rare Blood Types

Your blood type describes the antigens found on the surface of your RBCs. Blood typing and crossmatching are important for matching recipients with compatible blood units for blood transfusions and donated organs for transplants.

Blood typing identifies ABO and Rh type, which are of major concern for transfusion compatibility. In addition, RBCs have other antigen systems, such as Kell and Jk, for which most people have the typical form of those antigens. If you lack a common antigen, you may form antibodies (immune proteins) if exposed to the antigen via transfusion or pregnancy.

The presence of the antibody can be detected when your blood is crossmatched with donor blood before you receive a transfusion. Crossmatching is a lab test in which blood from the recipient and a donated blood unit are mixed and checked for a reaction. The lab professionals would then work to identify your rare minor blood type and ensure you receive compatible blood.

Blood type compatibility is crucial to prevent transfusion reactions, in which antibodies in the recipient attack the donated red blood cells. Red blood cell incompatibility may also occur when antibodies from the donor's plasma attack the recipient's RBC antigens.

Organ transplantation also involves matching compatible blood types and other antigens on donor and recipient organs, which can potentially affect the transplanted organ's success.

If you have an uncommon blood type, you may be less compatible with other blood types, and there may be fewer matched donors from whom you could receive a blood transfusion or organ donation.

In some situations, your healthcare providers might recommend an autologous blood transfusion if you have a rare blood type. This would involve donating your own blood before having a planned surgical procedure so that you can receive your own blood during the surgery. This process can ensure that you receive a match.

Health Concerns With Rh Null Blood Type

Since Rh proteins help maintain the structure of a healthy RBS, Rh_null causes the RBCs to become fragile and it is difficult to find a match for people who have this rare blood type when they need a blood donation.

If you have Rh null blood type, you can also be susceptible to hemolysis, a condition in which the RBCs break down faster than they are regenerated. Hemolysis can be dangerous, causing anemia (RBC deficiency) and organ damage from the RBC breakdown products.

Because potential blood donors for people with Rh null blood are rare, some solutions being examined include gene therapy or stem cells to help manage hemolysis and anemia in this population.

Other Blood Types

The rarest of the major blood types in the U.S. population are AB negative (0.6%), B negative (1.5%), and AB positive (3.4%). The most common of the ABO and Rh blood types is O positive.

Having Rh negative blood (which is different from Rh null) can be a health concern during pregnancy. If a pregnant person who has Rh negative blood type is carrying a fetus who has Rh positive blood type, the pregnant person could develop anti-Rh antibodies (immune proteins) during labor and delivery.

If the person giving birth subsequently has another pregnancy with an Rh positive fetus, they can produce antibodies to attack the fetus's blood, causing a life-threatening condition called hemolytic disease of the fetus and newborn. Fortunately, because this risk is well understood, preventative medications can be used to inhibit this reaction from occurring.

Variants in Rh proteins also complicate transfusions for people who need ongoing transfusion therapy, such as people with sickle cell disease. People of African ancestry are more likely to have Rh variations and the genes that produce sickle cell disease. Transfusion can result in producing unexpected Rh antibodies.

Blood Types by Population

The prevalence of ABO and Rh blood types varies among different populations. For example, in the U.S. population, the American Red Cross notes the A positive blood type is seen in approximately 33% of Caucasians, 24% of African Americans, 27% of Asians, and 29% of Latino Americans.

Keep in mind that even these labels are not consistent among U.S. epidemiological surveys and that many people have more than one of these identities or have a different identity that doesn't fall into any of these categories.

However, having a general idea of the prevalence of different blood types among different populations can help healthcare organizations as they direct blood drives or search for compatible organ donors.

Learn More: What Do Hospitals Do in a Blood Shortage?

How Is Blood Type Inherited?

Blood type is genetically determined, with the genes for the antigens of the ABO system inherited separately from those of the Rh system. Genes also determine the other antigen systems, and these are inherited separately.

Your blood type is a phenotype, an observed trait. It isn't obvious what your genotype is, which is the genetic combination that results in the phenotype.

For the ABO system, each parent may have genetic combinations of AA, AB, AO, BB, BO, or OO. In this system, O means no A or B antigen is produced, while A or B means the matching antigen is produced. The offspring inherits one gene from each parent, with their observed blood type dependent on which combination they end up with.

As parents with blood type A may have genetic combinations of AA or AO, and parents with blood type B may have genetic combinations of BB or BO, it's possible that a type A parent and type B parent have children of types A, B, or O.

Parents who are type O can only have the gene combination of OO. The blood type of their offspring will depend on the genetic combination of the other parent.

The Rh antigens are separately inherited, with each parent having a possible combination of two genes that express Rh(D), only one gene that expresses Rh(D), or no genes that express Rh(D). (This is a simplified explanation, as the Rh system can be complex.)

A person can inherit any combination of their parents’ blood types—so siblings do not necessarily have the same blood types. For example, an individual who has a parent with A negative blood type and a parent with B positive blood type can be A, B, or AB type with either positive or negative Rh types

Blood typing usually involves testing a blood sample for RBC surface antigens. However, there are several different subtypes of A, B, and Rh antigens. Sometimes, genetic testing can provide even more precise test results than routine blood typing.

Compatibility for Transfusions

Blood transfusions need to be typed and crossmatched to ensure safe transfusion. You can donate blood no matter what your blood type is, but not everyone can safely receive your blood donation. The blood types you can receive are limited based on your own blood type.

The key is that a person cannot receive blood from a donor who has antigens that the recipient does not have.

Related: Universal Blood Type Donors and Recipients

Summary

People who have the rarest blood type, called Rh null, do not have any Rh antigens on the RBCs. People with this very rare blood type cannot receive any blood transfusions from any other blood type. Additionally, having Rh null blood is associated with health complications, which means that you will need lifelong close medical care if you have this blood type.

There are many different blood types, and some are more common than others. Additionally, populations may have a higher or lower incidence of different blood types. Having a rare blood type can complicate finding a compatible blood transfusion.

Read the original article on Verywell Health.