The Quick Answer
Blood type is determined by the presence or absence of A and B antigens on red blood cells and is classified into four main groups -- A, B, AB, and O -- each with a positive or negative Rh factor. This produces eight common blood types: A+, A-, B+, B-, AB+, AB-, O+, and O-.
Your blood type is inherited from your parents through specific alleles and remains the same for your entire life. It matters most in two clinical settings: blood transfusions and pregnancy.
Disclaimer: This article is for educational purposes only and does not constitute medical advice. Consult a healthcare professional for questions about your blood type or health.
The ABO Blood Group System
The ABO system, discovered by Karl Landsteiner in 1901, classifies blood based on two antigens -- A and B -- found on the surface of red blood cells. Your body also produces antibodies against whichever antigens your red blood cells lack.
| Blood Type | Antigens on Red Blood Cells | Antibodies in Plasma |
|---|---|---|
| A | A antigen | Anti-B |
| B | B antigen | Anti-A |
| AB | Both A and B antigens | Neither (no antibodies) |
| O | Neither antigen | Both anti-A and anti-B |
This antigen-antibody relationship is why transfusion compatibility matters. If you receive blood with antigens your plasma has antibodies against, those antibodies will attack the foreign red blood cells, causing a potentially fatal hemolytic reaction.
The Rh Factor
The Rh system involves many antigens, but the most clinically important is the Rh D antigen. If the D antigen is present on your red blood cells, you are Rh positive (+). If absent, you are Rh negative (-).
Approximately 85% of the population is Rh positive. Combined with the ABO groups, this produces eight standard blood types.
Blood Type Frequency in the United States
| Blood Type | Approximate Frequency |
|---|---|
| O+ | 37% |
| A+ | 36% |
| B+ | 8% |
| AB+ | 3% |
| O- | 7% |
| A- | 6% |
| B- | 1% |
| AB- | 1% |
Source: American Red Cross. Frequencies vary by ethnic group and geographic region.
How Blood Types Are Inherited
Blood type inheritance follows Mendelian genetics. The ABO gene has three alleles:
- IA -- produces the A antigen (dominant over i)
- IB -- produces the B antigen (dominant over i)
- i -- produces no antigen (recessive)
IA and IB are codominant with each other, meaning if you inherit one of each, both antigens are expressed (type AB). Both are dominant over i.
Every person inherits one allele from each parent, giving six possible genotypes:
| Genotype | Blood Type |
|---|---|
| IA IA or IA i | Type A |
| IB IB or IB i | Type B |
| IA IB | Type AB |
| i i | Type O |
Punnett Square Example: Father AO x Mother BO
A father with blood type A and genotype IA i, and a mother with blood type B and genotype IB i:
| Father: IA | Father: i | |
|---|---|---|
| Mother: IB | IA IB (Type AB) | IB i (Type B) |
| Mother: i | IA i (Type A) | i i (Type O) |
Result: 25% Type AB, 25% Type A, 25% Type B, 25% Type O.
This is why two parents with types A and B can produce a child with any of the four blood types -- but only if both carry the recessive i allele.
Inheritance Outcomes by Parent Combination
| Parent 1 | Parent 2 | Possible Child Types |
|---|---|---|
| O (ii) | O (ii) | O only |
| O (ii) | A (IA i) | A or O |
| O (ii) | B (IB i) | B or O |
| A (IA i) | A (IA i) | A or O |
| A (IA i) | B (IB i) | A, B, AB, or O |
| A (IA IA) | B (IB IB) | AB only |
| AB (IA IB) | O (ii) | A or B |
| AB (IA IB) | AB (IA IB) | A, B, or AB (not O) |
Note: Outcomes depend on whether the parent's genotype is homozygous (e.g., IA IA) or heterozygous (e.g., IA i).
The Rh factor is inherited separately. The Rh positive allele (D) is dominant over Rh negative (d). Two Rh positive parents who are both heterozygous (Dd) have a 25% chance of an Rh negative child.
Blood Transfusion Compatibility
The fundamental rule of transfusion: a recipient must not receive red blood cells carrying antigens they have antibodies against.
Red Blood Cell Compatibility Matrix
| Recipient | Can Receive From |
|---|---|
| O- | O- only |
| O+ | O-, O+ |
| A- | A-, O- |
| A+ | A+, A-, O+, O- |
| B- | B-, O- |
| B+ | B+, B-, O+, O- |
| AB- | AB-, A-, B-, O- |
| AB+ | All types (universal recipient) |
O negative is the universal red blood cell donor because O- cells carry no A, B, or Rh D antigens.
AB positive is the universal recipient because AB+ plasma contains no anti-A, anti-B, or anti-D antibodies, so it will not attack any donor red blood cells.
For plasma transfusions, the rules reverse: AB plasma is the universal donor (no antibodies to transfer), while O plasma contains both anti-A and anti-B antibodies.
Rh Incompatibility in Pregnancy
Rh incompatibility becomes a concern when an Rh negative mother carries an Rh positive fetus (inheriting the D allele from the father). If fetal red blood cells enter the mother's bloodstream -- typically during delivery, miscarriage, or certain procedures -- her immune system may produce anti-D antibodies.
This usually does not affect the first pregnancy. However, in subsequent pregnancies with an Rh positive fetus, the mother's existing anti-D antibodies can cross the placenta and attack the baby's red blood cells, causing hemolytic disease of the fetus and newborn (HDFN).
This condition is preventable. An injection of Rh immunoglobulin (RhIg), commonly known as RhoGAM, is given to Rh negative mothers at around 28 weeks of pregnancy and within 72 hours of delivery. RhIg prevents the mother's immune system from producing anti-D antibodies. According to the American College of Obstetricians and Gynecologists, this treatment has made severe HDFN rare in developed countries.
How Blood Type Is Determined
Blood typing is performed using a simple test called forward typing. A blood sample is mixed with anti-A serum and anti-B serum:
- Clumping with anti-A only = Type A
- Clumping with anti-B only = Type B
- Clumping with both = Type AB
- Clumping with neither = Type O
A separate test with anti-D serum determines Rh status. The entire process takes minutes and is standard before any blood transfusion.
Frequently Asked Questions
What blood type will my child have?
A child's blood type depends on both parents' genotypes. For example, if one parent is AO (type A) and the other is BO (type B), the child could be AB, A, B, or O with equal 25% probability. Use the inheritance table above to check possible outcomes for specific parent combinations.
Why is O negative the universal donor?
O negative red blood cells lack A antigens, B antigens, and the Rh D antigen. This means they will not trigger an immune reaction in any recipient regardless of blood type, making O negative safe for emergency transfusions when the recipient's type is unknown.
Can your blood type change?
Under normal circumstances, no. Blood type is genetically determined and fixed for life. In extremely rare cases, a bone marrow transplant from a donor with a different blood type can change the recipient's blood type, because the new marrow produces blood cells with the donor's antigens.
Which blood type is the rarest?
AB negative is the rarest of the eight common blood types, found in roughly 1% of the US population. O positive is the most common at about 37%.
What is the Rh factor?
The Rh factor refers to the presence or absence of the Rh D antigen on red blood cells. If the antigen is present, the blood type is Rh positive (+). If absent, it is Rh negative (-). About 85% of people are Rh positive.
Can two type O parents have a type A child?
No. Two type O parents both have the genotype ii (two recessive alleles). They can only pass the i allele to their children, so all children will be type O.
What happens if you receive the wrong blood type?
Receiving incompatible blood triggers an acute hemolytic transfusion reaction. The recipient's antibodies attack the donor red blood cells, causing them to clump and break apart. This can lead to kidney failure, shock, and can be fatal.
Is blood type related to diet or personality?
There is no credible scientific evidence supporting blood type diets or blood type personality theories. A 2013 systematic review in the American Journal of Clinical Nutrition found no evidence to validate the blood type diet hypothesis.
Why do hospitals need to know your blood type?
Hospitals determine blood type before transfusions to ensure compatibility and prevent potentially fatal immune reactions. They also check blood type during pregnancy to identify Rh incompatibility risk between mother and fetus.
Can a father's blood type rule out paternity?
Blood type can exclude paternity in some cases but cannot confirm it. For example, two type O parents cannot produce a type AB child. However, modern DNA testing is far more accurate for paternity determination.
Try the Calculator
Use the blood type calculator to predict possible blood types of children based on parent blood types, or explore inheritance probabilities with the probability calculator.