Your blood type is determined by two main genetic systems: the ABO blood group and the Rh factor. Each person inherits one ABO gene from each parent, producing either type A, B, AB, or O. The Rh factor comes from a separate gene, and if you inherit the RhD antigen, you are Rh positive; if not, you are Rh negative. Together, these genes create the eight common blood types: A+, A−, B+, B−, AB+, AB−, O+, and O−.
What determines ABO blood type?
The ABO blood type comes from a single gene on chromosome 9. This gene has three main versions, or alleles: A, B, and O.
The A and B alleles each produce a specific enzyme that adds a sugar molecule to the surface of red blood cells. These sugars are called antigens. The A allele makes the A antigen. The B allele makes the B antigen. The O allele does not produce a working enzyme, so no A or B antigen is added.
You inherit one allele from each parent. If you get an A from one parent and a B from the other, your blood cells have both A and B antigens — that is type AB. If you get an A and an O, you have only A antigens — type A. A B and an O gives type B. Two O alleles gives type O — no A or B antigens.
Type O is the most common blood type worldwide, though the distribution varies by region. The A and B antigens are part of your body’s immune identity. Your immune system treats the antigens it does not have as foreign. That is why blood transfusions must be matched carefully.
How does the Rh factor work?
The Rh factor is controlled by a separate gene on chromosome 1. The main part of this system is a protein called RhD. If your red blood cells have the RhD protein, you are Rh positive. If they lack it, you are Rh negative.
Like ABO, you inherit one Rh gene from each parent. The Rh positive version (Rh+) is dominant. That means if you inherit even one Rh+ gene, you will be Rh positive. You are Rh negative only if you inherit two Rh− genes — one from each parent.
About 85% of people in the United States are Rh positive. The Rh negative type is less common but varies by population. In some regions of the world, Rh negative is very rare.
Rh factor becomes especially important during pregnancy. If an Rh negative mother carries an Rh positive baby, her immune system can make antibodies against the RhD protein. This usually does not harm the first pregnancy, but it can affect later pregnancies. A medication called Rh immunoglobulin given during and after pregnancy prevents this problem.
What causes different blood types ABO genes and Rh factor?
The simple answer is genetics. Your ABO blood type and Rh factor are inherited from your parents through specific genes. The ABO gene on chromosome 9 has three common alleles — A, B, and O — and your combination determines whether you make A antigens, B antigens, both, or neither. The RhD gene on chromosome 1 determines whether you produce the RhD protein. The dominant Rh+ allele produces the protein; the recessive Rh− allele does not.
These two genetic systems are inherited independently. That means your ABO type and your Rh type are not linked. You can have any ABO type with either Rh positive or Rh negative.
Mutations in these genes are rare but can produce unusual blood types. For example, the Bombay phenotype (hh) prevents the building of A or B antigens even if the ABO gene is normal. These individuals appear as type O but cannot safely receive type O blood because their immune system reacts to other antigens present in most red blood cells.
Why do blood types matter for transfusions?
Blood transfusion works only if the recipient’s immune system does not attack the donor’s red blood cells. The ABO and Rh systems are the most important for matching.
If you have type A blood, your immune system makes antibodies against the B antigen. If you receive type B or type AB blood, those antibodies attack the foreign red cells, causing a transfusion reaction. Type O blood has no A or B antigens, so it can be given to anyone in small amounts — that is why O negative is called the universal donor.
Rh matching also matters. An Rh negative person can develop antibodies if they receive Rh positive blood. Once those antibodies form, a future transfusion with Rh positive blood can cause a reaction. For this reason, blood banks match Rh type whenever possible, especially for women of childbearing age.
There are over 300 other blood group systems, but ABO and Rh cause the most frequent and severe transfusion reactions. Hospitals always check both before transfusion.
Can your blood type change?
Your blood type is set at birth and does not change naturally. However, there are rare exceptions.
A bone marrow transplant replaces your blood‑forming stem cells with a donor’s cells. Over time, the recipient’s red blood cells may acquire the donor’s blood type. This takes weeks to months and is a planned part of the transplant process.
Some medical conditions can temporarily alter how blood types are detected. For example, certain cancers or infections can reduce antigen expression on red blood cells, making a person appear as a different blood type on lab tests. This is not a true change — the original genes are still there.
Pregnancy does not change a woman’s blood type. Claims that blood type can shift with age or diet are not supported by evidence.
What other blood group systems exist?
Besides ABO and Rh, dozens of other blood group systems have been identified. The most clinically important ones after ABO and Rh are Kell, Duffy, Kidd, and MNS.
Kell antigens can cause transfusion reactions and hemolytic disease of the newborn. Duffy antigens are also targets for malaria parasites — people who lack certain Duffy antigens have natural resistance to some types of malaria. Kidd antigens can cause delayed transfusion reactions.
These systems are less likely to cause problems because most people lack naturally occurring antibodies against them. But after a transfusion or pregnancy, a person can develop antibodies to any of these antigens. Blood banks routinely screen for common antibodies before cross‑matching blood.
The complete picture of your blood type includes all these systems, but for everyday medical care, ABO and Rh remain the standard.
Frequently Asked Questions
Can my blood type change over time?
No, your blood type is genetically determined and does not change naturally. The only exception is after a bone marrow transplant, when the recipient may eventually produce the donor’s blood type.
What is the rarest blood type?
The rarest of the common blood types is AB negative, found in less than 1% of people in the United States. Some uncommon blood types in other systems, such as the Bombay phenotype, are even rarer.
Does blood type affect personality or diet?
No scientific evidence supports the idea that blood type influences personality or determines which foods you should eat. Blood type diets and personality systems are not backed by reliable clinical studies.
Why is Rh negative important during pregnancy?
If an Rh negative mother carries an Rh positive baby, her immune system may make antibodies that attack the baby’s red blood cells in a future pregnancy. Rh immunoglobulin shots prevent this from happening.
