Blood Type Compatibility Calculator

What is Blood Type?

Blood type (also called blood group) is a classification of blood based on the presence or absence of specific inherited antigens on the surface of red blood cells (RBCs). These antigens are proteins and sugars that coat the surface of every red blood cell in your body. Your immune system uses these antigens to recognize which blood cells belong in your body and which do not.

The two most important blood group systems for blood transfusions are the ABO system and the Rh factor system. Together, they create the eight common blood types: A+, A-, B+, B-, AB+, AB-, O+, and O-. Understanding your blood type is critically important for safe blood transfusions, organ transplants, and managing pregnancy complications.

The ABO system classifies blood based on the presence of two antigens (A and B) on the surface of red blood cells. If your red blood cells have the A antigen, you have type A blood. If they have the B antigen, you have type B. If both A and B antigens are present, your blood type is AB. If neither antigen is present, you have type O blood. In addition, your plasma naturally contains antibodies against the antigens you lack: type A blood has anti-B antibodies, type B has anti-A antibodies, type O has both anti-A and anti-B, and type AB has neither.

The Rh factor refers to the presence or absence of the RhD antigen (also called the D antigen) on the red blood cell surface. If the RhD antigen is present, you are Rh-positive (+). If it is absent, you are Rh-negative (-). This gives us the positive and negative distinction within each ABO blood type.

Blood Type Compatibility for Red Blood Cells

When it comes to red blood cell transfusions, the key rule is that the recipient must not have antibodies that will attack the donor's red blood cells. This means the donor's red blood cells must not carry antigens that the recipient's immune system will recognize as foreign.

Here is the complete red blood cell compatibility matrix:

O- (Universal Donor): Can donate red blood cells to everyone: O-, O+, A-, A+, B-, B+, AB-, AB+. This is because O- red blood cells have no A, B, or RhD antigens, so no recipient's antibodies will attack them. O- blood is critically important in emergency situations when there is no time to determine a patient's blood type.
O+: Can donate to O+, A+, B+, AB+. The O+ red blood cells lack A and B antigens but have the RhD antigen, so they can only go to Rh-positive recipients.
A-: Can donate to A-, A+, AB-, AB+. A- red blood cells carry the A antigen but not RhD, so they are compatible with recipients who have the A antigen or AB (who will not attack A), as long as the Rh factor is accounted for (Rh-negative can give to both Rh-positive and Rh-negative).
A+: Can donate to A+ and AB+. A+ red blood cells carry both A and RhD antigens, limiting recipients to those who are A or AB and Rh-positive.
B-: Can donate to B-, B+, AB-, AB+. Similar logic to A-: the B antigen is compatible with B and AB recipients, and Rh-negative can give to both Rh-positive and Rh-negative.
B+: Can donate to B+ and AB+. B+ carries both B and RhD antigens, restricting recipients to B+ and AB+.
AB-: Can donate to AB- and AB+. AB- red blood cells carry both A and B antigens (but not RhD), so only AB recipients (who have no anti-A or anti-B antibodies) can accept them.
AB+: Can donate to AB+ only. AB+ red blood cells carry A, B, and RhD antigens, making them compatible only with AB+ recipients.

For red blood cell reception, the rules are reversed. Each blood type can receive from donors whose red blood cells will not be attacked by the recipient's antibodies:

O-: Can only receive from O-. Since O- plasma contains anti-A, anti-B antibodies, and the recipient is Rh-negative, only O- red blood cells (with no antigens) are safe.
O+: Can receive from O- and O+.
A-: Can receive from A- and O-.
A+: Can receive from A+, A-, O+, O-.
B-: Can receive from B- and O-.
B+: Can receive from B+, B-, O+, O-.
AB-: Can receive from AB-, A-, B-, O-.
AB+ (Universal Recipient): Can receive from everyone: AB+, AB-, A+, A-, B+, B-, O+, O-. AB+ individuals have no anti-A, anti-B, or anti-RhD antibodies, so their immune system will not attack any donated red blood cells.

Blood Type Compatibility for Plasma

Plasma compatibility follows essentially the opposite rules of red blood cell compatibility. This is because plasma contains antibodies rather than antigens. When donating plasma, you want to ensure the donor's antibodies will not attack the recipient's red blood cells.

For plasma donations, the ABO group matters but the Rh factor is generally not significant (since plasma does not contain red blood cells and therefore no RhD antigens). The key rules are:

AB plasma (Universal Plasma Donor): AB plasma contains neither anti-A nor anti-B antibodies, so it can be safely given to anyone. This is why AB plasma is especially valuable and is often used in emergency situations.
A plasma: Contains anti-B antibodies, so it can only be given to recipients with type A or type O blood (who do not have B antigens on their red blood cells).
B plasma: Contains anti-A antibodies, so it can only be given to recipients with type B or type O blood (who do not have A antigens).
O plasma: Contains both anti-A and anti-B antibodies, so it can only be given to type O recipients (who have neither A nor B antigens).

For plasma reception, the logic is similarly reversed:

O recipients (Universal Plasma Recipient): Since O red blood cells have no A or B antigens, plasma from any ABO type is safe. The antibodies in the received plasma will have nothing to attack.
A recipients: Can receive plasma from A and AB donors (plasma without anti-A antibodies).
B recipients: Can receive plasma from B and AB donors (plasma without anti-B antibodies).
AB recipients: Can only receive plasma from AB donors, because A, B, and O plasma all contain antibodies that would attack AB red blood cells (which carry both A and B antigens).

Understanding plasma compatibility is crucial in trauma and surgical settings, where large volumes of plasma may be transfused to manage bleeding and clotting disorders.

Universal Donor: O Negative

O negative (O-) blood is often referred to as the "universal donor" type for red blood cell transfusions. This designation exists because O- red blood cells lack all three major antigens (A, B, and RhD), meaning they will not trigger an immune response in any recipient regardless of their blood type.

O- blood is critically important in emergency medicine. When a patient arrives at an emergency room with severe bleeding and there is no time to determine their blood type (a process called typing and crossmatching), O- blood is the default choice. Trauma centers and emergency helicopters typically carry O- blood for this exact reason.

Despite its immense importance, O- blood is relatively rare, found in only about 6.6% of the general population. This creates a constant challenge for blood banks, which must maintain adequate supplies of O- blood while it is simultaneously the most in-demand type. Blood donation centers frequently put out special appeals for O- donors because demand consistently outpaces supply.

It is worth noting that while O- is the universal donor for red blood cells, O- plasma is actually the most restrictive: it contains both anti-A and anti-B antibodies and can only be given to O recipients. For plasma donation, AB is the universal donor type.

O- donors can only receive red blood cells from other O- donors, making them the most restrictive recipients. This is an important consideration for O- individuals to understand, as they should be aware that in a transfusion scenario, only O- blood is compatible for them.

Universal Recipient: AB Positive

AB positive (AB+) blood is called the "universal recipient" type for red blood cell transfusions. AB+ individuals have all three major antigens (A, B, and RhD) on their red blood cells and, crucially, their plasma contains none of the corresponding antibodies (no anti-A, no anti-B, and no anti-RhD). This means their immune system will not attack red blood cells from any donor, regardless of the donor's blood type.

AB+ recipients can safely receive red blood cells from all eight blood types: O-, O+, A-, A+, B-, B+, AB-, and AB+. This makes AB+ individuals the most flexible recipients in terms of blood transfusion options.

However, AB+ is one of the rarest blood types, found in only about 3.4% of the population. While AB+ individuals benefit from being universal recipients for red blood cells, they face a different situation with plasma: AB recipients can only receive plasma from AB donors, making them the most restricted plasma recipients.

Conversely, AB plasma is the universal plasma donor type because it contains no anti-A or anti-B antibodies. This is why blood banks particularly value AB donors for platelet and plasma donations, even though their red blood cell donations are the most restrictive (AB+ can only donate red blood cells to other AB+ individuals).

The ABO Blood Group System

The ABO blood group system is the most important blood classification system used in human blood transfusion. It was discovered in 1901 by Austrian physician Karl Landsteiner, who received the Nobel Prize in Physiology or Medicine in 1930 for this groundbreaking work. Before Landsteiner's discovery, blood transfusions were extremely dangerous and often fatal because physicians did not understand why some transfusions succeeded while others caused deadly reactions.

The ABO system is based on two antigens -- carbohydrate molecules called A and B -- that are found on the surface of red blood cells. These antigens are built by enzymes that add specific sugar molecules to a precursor molecule called the H antigen (which is present on all red blood cells regardless of blood type).

The four ABO blood groups are determined by which antigens are present:

Type A: The A enzyme adds N-acetylgalactosamine to the H antigen, creating the A antigen. Type A plasma contains anti-B antibodies (also called isohemagglutinins).
Type B: The B enzyme adds D-galactose to the H antigen, creating the B antigen. Type B plasma contains anti-A antibodies.
Type AB: Both enzymes are active, creating both A and B antigens. Type AB plasma contains neither anti-A nor anti-B antibodies.
Type O: Neither enzyme is functional (due to a deletion in the gene), so only the unmodified H antigen is present. Type O plasma contains both anti-A and anti-B antibodies.

The antibodies in the ABO system are unusual in that they are "naturally occurring" -- they develop in the first few months of life without any previous exposure to incompatible blood. This is thought to occur because A-like and B-like antigens are common on bacteria and food substances that infants encounter early in life. The immune system generates antibodies against the antigens that are not present on the individual's own red blood cells.

The ABO blood group is determined by a single gene located on chromosome 9 (specifically at position 9q34.2). This gene has three main alleles: I^A, I^B, and i. The I^A and I^B alleles are codominant (both are expressed when present together, producing type AB), while the i allele is recessive. A person inherits one allele from each parent, giving six possible genotype combinations that produce four phenotypes (the observable blood types).

The Rh Factor

The Rh blood group system is the second most important blood group system after ABO. It is named after the Rhesus monkey, in which a related antigen was first discovered in 1940 by Karl Landsteiner and Alexander Wiener. The Rh system is considerably more complex than ABO, involving over 50 different antigens, but the most significant by far is the D antigen (RhD).

When we refer to someone as "Rh-positive" or "Rh-negative" (the + or - in blood type designations like A+ or O-), we are referring specifically to the presence or absence of the RhD antigen on the surface of their red blood cells. Approximately 85% of the global population is Rh-positive (they have the D antigen), while about 15% is Rh-negative (they lack the D antigen). The frequency varies significantly by ethnicity: about 85% of Caucasians, 95% of African Americans, and over 99% of East Asians are Rh-positive.

Unlike the ABO system, anti-RhD antibodies are not naturally occurring. Rh-negative individuals do not spontaneously develop anti-D antibodies. Instead, these antibodies only form after an Rh-negative person is exposed to Rh-positive blood, either through a blood transfusion or during pregnancy when an Rh-negative mother carries an Rh-positive fetus. This process is called alloimmunization or sensitization.

The RhD gene (also called RHD) is located on chromosome 1 (at position 1p36.11). Rh-positive individuals have at least one functional copy of the RHD gene, while Rh-negative individuals in most populations have a complete deletion of the RHD gene. The inheritance pattern is straightforward: the Rh-positive allele (D) is dominant over the Rh-negative allele (d). So individuals with genotypes DD or Dd are Rh-positive, while only dd individuals are Rh-negative.

Other important Rh antigens besides D include C, c, E, and e. These can also cause transfusion reactions and are considered during detailed blood typing (extended phenotyping), particularly for patients who require frequent transfusions, such as those with sickle cell disease or thalassemia.

Blood Type Inheritance

Blood type is inherited genetically, following the fundamental principles of Mendelian genetics. Each person inherits one ABO allele from their mother and one from their father, as well as one Rh allele from each parent. Understanding these inheritance patterns can help predict the possible blood types of children based on their parents' blood types.

ABO Inheritance: The ABO gene has three alleles: I^A (produces A antigen), I^B (produces B antigen), and i (produces no antigen). I^A and I^B are codominant with each other, and both are dominant over i. The possible genotypes and resulting phenotypes are:

Type A: Genotype I^AI^A or I^Ai
Type B: Genotype I^BI^B or I^Bi
Type AB: Genotype I^AI^B
Type O: Genotype ii

Example Punnett Square: If one parent is type A (genotype I^Ai) and the other is type B (genotype I^Bi), the possible offspring blood types are: type A (I^Ai), type B (I^Bi), type AB (I^AI^B), or type O (ii) -- each with a 25% probability. This is one of the rare parent combinations that can produce children of all four ABO blood types.

Rh Inheritance: The Rh factor follows simpler dominant/recessive genetics. Let D represent the dominant Rh-positive allele and d represent the recessive Rh-negative allele:

Two Rh-positive parents (both Dd) can have an Rh-negative child (dd) with a 25% probability.
Two Rh-negative parents (both dd) will always have Rh-negative children.
An Rh-positive parent (DD) and an Rh-negative parent (dd) will always have Rh-positive children (all Dd).

Blood type inheritance has historically been used in paternity testing (before DNA testing became available) and in forensic science. While blood type alone cannot prove paternity, it can definitively exclude certain individuals. For example, two parents who are both type O cannot have a child with type A, B, or AB blood.

Blood Type Distribution by Ethnicity and Region

The frequency of different blood types varies significantly across geographic regions and ethnic groups. These variations are the result of evolutionary pressures, genetic drift, population migrations, and possibly natural selection related to resistance to certain diseases.

Global averages (approximate, based on donors in the United States):

O+: 37.4% | O-: 6.6%
A+: 35.7% | A-: 6.3%
B+: 8.5% | B-: 1.5%
AB+: 3.4% | AB-: 0.6%

Regional variations include:

South America: Type O is overwhelmingly dominant. Among indigenous populations in Central and South America, type O approaches nearly 100%. In some indigenous groups, blood types A, B, and AB are virtually nonexistent.
Europe: Type A is relatively common, especially in Scandinavia and Central Europe, where it can reach 40-45% of the population. Type B is less common (8-15%). Rh-negative blood is more prevalent in Europe than anywhere else, particularly among the Basque people of Spain and France, where Rh-negative frequency can reach 30-35%.
East Asia: Type B is more common than in Western populations (20-30%). Type O is somewhat less common. Rh-negative blood is extremely rare (less than 1% in some East Asian populations).
Sub-Saharan Africa: Type O is the most common, and type B is more frequent than in European populations. Rh-negative blood is relatively rare (about 5%).
South Asia: Type B is quite common (30-35% in parts of India). Type O and A are also present in significant proportions.

These distributions have practical implications for blood banking. In regions where certain blood types are rare, maintaining adequate supplies can be challenging. International blood type registries and rare blood banks exist to help locate compatible donors for patients with uncommon blood types or rare antigen combinations.

Why Blood Type Matters in Transfusions

Blood type compatibility is literally a matter of life and death in transfusion medicine. When incompatible blood is transfused, the recipient's immune system recognizes the foreign antigens on the donated red blood cells and launches a devastating immune attack. This is known as a hemolytic transfusion reaction.

In an acute hemolytic transfusion reaction (the most severe type), the recipient's pre-existing antibodies immediately bind to the incompatible donor red blood cells. This triggers a cascade of immune responses:

Complement activation: The antibody-antigen complexes activate the complement system, a group of proteins in the blood that can directly destroy red blood cells by punching holes in their membranes (intravascular hemolysis).
Massive red blood cell destruction: The rapid destruction of donor red blood cells releases large quantities of hemoglobin into the bloodstream (hemoglobinemia) and urine (hemoglobinuria, which turns urine dark brown or red).
Inflammatory response: The destruction triggers a systemic inflammatory response, causing fever, chills, back pain, chest tightness, and a sense of impending doom.
Coagulation cascade: The reaction can trigger disseminated intravascular coagulation (DIC), a life-threatening condition in which blood clots form throughout the body, simultaneously consuming clotting factors and causing uncontrollable bleeding.
Kidney failure: Free hemoglobin and the products of red blood cell destruction can damage the kidneys, potentially leading to acute renal failure.
Shock and death: In severe cases, the combined effects of hemolysis, DIC, and kidney failure can lead to cardiovascular collapse, multi-organ failure, and death.

ABO-incompatible transfusions are the most dangerous because the anti-A and anti-B antibodies in the ABO system are "naturally occurring" IgM antibodies that are highly efficient at activating complement and causing rapid intravascular hemolysis. Even a small amount of ABO-incompatible blood (as little as 10-50 mL) can cause a fatal reaction.

To prevent transfusion reactions, blood banks perform rigorous testing before every transfusion. This includes ABO and Rh typing (determining the patient's blood type), antibody screening (checking for unexpected antibodies in the patient's plasma), and crossmatching (mixing a sample of the patient's plasma with the donor's red blood cells to check for any reaction). These safety measures have made transfusion reactions rare in modern medicine, but human error (such as mislabeling blood samples or administering blood to the wrong patient) remains the leading cause of fatal ABO-incompatible transfusions.

Rh Incompatibility in Pregnancy

Rh incompatibility in pregnancy, also known as Rh disease or hemolytic disease of the fetus and newborn (HDFN), occurs when an Rh-negative mother carries an Rh-positive fetus. This is only possible when the father is Rh-positive (either DD or Dd genotype). The condition was a significant cause of infant mortality before the development of preventive treatment in the 1960s.

How Rh disease develops:

Sensitization: During pregnancy (especially during delivery), small amounts of the fetus's Rh-positive blood can enter the mother's Rh-negative bloodstream. This is called fetomaternal hemorrhage. The mother's immune system recognizes the RhD antigen as foreign and produces anti-D antibodies. This first exposure is called sensitization, and it typically does not harm the first Rh-positive baby because the immune response is slow (it takes time to produce sufficient antibodies).
Second pregnancy: In subsequent pregnancies with an Rh-positive fetus, the mother's immune system "remembers" the RhD antigen and rapidly produces large quantities of anti-D antibodies (IgG class, which can cross the placenta). These antibodies enter the fetal bloodstream and attack the fetus's Rh-positive red blood cells.
Fetal hemolytic anemia: The destruction of fetal red blood cells leads to anemia. In mild cases, the fetus compensates by producing more red blood cells. In severe cases, the fetus can develop heart failure, fluid accumulation (hydrops fetalis), brain damage from severe anemia, or even death in utero.

Prevention with RhoGAM: Rh disease is now almost entirely preventable thanks to Rh immune globulin (RhIG), commonly known by the brand name RhoGAM. This is an injection of anti-D antibodies given to Rh-negative mothers at specific times during pregnancy and after delivery.

RhoGAM works by binding to any fetal RhD-positive red blood cells that have entered the mother's bloodstream and destroying them before her immune system can detect them and mount an immune response. This prevents sensitization and protects future pregnancies. The standard protocol includes:

An injection at approximately 28 weeks of gestation (prophylactic dose)
An injection within 72 hours after delivery of an Rh-positive baby
Additional doses after any event that might cause fetomaternal hemorrhage (miscarriage, amniocentesis, abdominal trauma, etc.)

Since the introduction of RhoGAM in 1968, the incidence of Rh disease has decreased dramatically. However, it remains a concern in developing countries where access to RhoGAM may be limited, and in cases where the preventive treatment is not administered properly.

Frequently Asked Questions

What is the rarest blood type?

AB negative (AB-) is the rarest of the eight common blood types, found in only about 0.6% of the population. AB+ is also relatively rare at 3.4%. Among the major blood types, O+ is the most common at 37.4%, followed closely by A+ at 35.7%. However, it is worth noting that beyond the ABO and Rh systems, there are over 300 other blood group antigens, and some extremely rare antigen combinations may be found in only a handful of people worldwide. These individuals may need blood from donors with the same rare antigen profile, which is managed through international rare blood registries.

Can my blood type change?

Under normal circumstances, your blood type does not change -- it is determined by your genetics and remains the same throughout your life. However, there are rare exceptions. A bone marrow or stem cell transplant from a donor with a different blood type will cause the recipient's blood type to gradually change to match the donor's type, because the new bone marrow produces new red blood cells with the donor's antigens. There have also been extremely rare case reports of blood type appearing to change temporarily during severe infections or certain cancers, but these situations involve unusual modifications to the antigens rather than true genetic changes.

Does blood type affect my health?

Research has found some statistical associations between blood type and certain health conditions, though the effects are generally small. Type O individuals appear to have a slightly lower risk of heart disease, deep vein thrombosis, and certain cancers (particularly pancreatic and gastric cancer), but a somewhat higher risk of peptic ulcers and bleeding disorders. Type A individuals may have a slightly elevated cardiovascular risk. Type B and AB individuals have been associated with different risk profiles in various studies. It is important to emphasize that these are statistical associations from population-level studies, and blood type is just one of many factors that influence disease risk. Lifestyle factors such as diet, exercise, smoking, and stress management have a far greater impact on health than blood type.

How often can I donate blood?

The recommended waiting period between whole blood donations is typically 56 days (8 weeks) in the United States, though this can vary by country. For platelet donations (apheresis), you can donate more frequently -- typically every 7 days, up to 24 times per year. Plasma donations can generally be made every 28 days. Double red blood cell donations require a longer waiting period of 112 days (16 weeks). These intervals exist to allow your body to replenish the donated blood components. Most healthy adults who weigh at least 110 pounds (50 kg) and are at least 16-17 years old (depending on local regulations) are eligible to donate blood. Certain medical conditions, medications, travel history, and other factors may temporarily or permanently defer someone from donating.

What is the difference between whole blood donation and apheresis?

Whole blood donation is the most common type. About one pint (approximately 470 mL) of blood is collected, which is then separated into its components (red blood cells, plasma, and platelets) at a processing lab. The entire donation process takes about 10-15 minutes for the actual blood draw (plus registration and screening time). Apheresis (also called component donation) uses a special machine that draws blood, separates it into components, retains the needed component (such as platelets, plasma, or red blood cells), and returns the remaining components to the donor. Apheresis takes longer (1-2 hours for platelets) but allows collection of larger quantities of a specific component. Platelet apheresis is particularly important for cancer patients who need frequent platelet transfusions.

Why is O negative blood used in emergencies?

O negative blood is used in emergencies because it is the universal red blood cell donor type. O- red blood cells have no A, B, or RhD antigens on their surface, which means they will not trigger an immune reaction in any recipient, regardless of the recipient's blood type. In emergency situations such as trauma, massive hemorrhage, or when a patient is unconscious and their blood type is unknown, there is no time to perform typing and crossmatching (which can take 30-60 minutes). Transfusing O- blood is considered safe as a life-saving measure until the patient's actual blood type can be determined. This is why O- blood is always in high demand and is one of the first types to run low in blood bank inventories during mass casualty events.

Can parents with the same blood type have a child with a different blood type?

Yes, absolutely. Because blood type is determined by genotype (the combination of alleles), not just phenotype (the observable blood type), parents with the same blood type can have children with different blood types. For example, two parents who are both type A could have genotypes I^Ai. Their children could be type A (I^AI^A or I^Ai) or type O (ii). Similarly, two type B parents (both I^Bi) could have a type O child. Two Rh-positive parents (both Dd) have a 25% chance of having an Rh-negative child (dd). The only blood types that are "true breeding" are type O (since both parents must be ii) and homozygous types (I^AI^A, I^BI^B, DD), but carriers (heterozygous individuals) can produce children with unexpected blood types.