What determines the blood type of a person

Blood Types Explained - A, B, AB and O

How Blood Type Is Determined And Why You Need To Know

Blood types are determined by the presence or absence of certain antigens – substances that can trigger an immune response if they are foreign to the body.  Since some antigens can trigger a patient's immune system to attack the transfused blood, safe blood transfusions depend on careful blood typing and cross-matching. Do you know what blood type is safe for you if you need a transfusion?

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There are four major blood groups determined by the presence or absence of two antigens, A and B, on the surface of red blood cells. In addition to the A and B antigens, there is a protein called the Rh factor, which can be either present (+) or absent (–), creating the 8 most common blood types (A+, A-,  B+, B-,  O+, O-,  AB+, AB-).



There are very specific ways in which blood types must be matched for a safe transfusion. The right blood transfusion can mean the difference between life and death. Use the interactive graphic below to learn more about matching blood types for transfusions.

Also, Rh-negative blood is given to Rh-negative patients, and Rh-positive or Rh-negative blood may be given to Rh-positive patients. The rules for plasma are the reverse. 

  • The universal red cell donor has Type O negative blood.
  • The universal plasma donor has Type AB blood.

There are more than 600 other known antigens, the presence or absence of which creates "rare blood types."  Certain blood types are unique to specific ethnic or racial groups. That’s why an African-American blood donation may be the best hope for the needs of patients with sickle cell disease, many of whom are of African descent.  Learn about blood and diversity.

Universal donors are those with an O negative blood type.  Why?  O negative blood can be used in transfusions for any blood type.  

Type O is routinely in short supply and in high demand by hospitals – both because it is the most common blood type and because type O negative blood is the universal blood type needed for emergency transfusions and for immune deficient infants.

Approximately 45 percent of Caucasians are type O (positive or negative), but 51 percent of African-Americans and 57 percent of Hispanics are type O. Minority and diverse populations, therefore, play a critical role in meeting the constant need for blood.

Types O negative and O positive are in high demand.  Only 7% of the population are O negative. However, the need for O negative blood is the highest because it is used most often during emergencies. The need for O+ is high because it is the most frequently occurring blood type (37% of the population).

The universal red cell donor has Type O negative blood. The universal plasma donor has Type AB blood. For more about plasma donation, visit the plasma donation facts.

Nearly 16 million blood components are transfused each year in the U.S."
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There are more than 600 other known antigens, the presence or absence of which creates "rare blood types."  Your blood type is considered rare if you lack antigens that 99% of the people are positive for. If you somehow lack an antigen that 99.99% are positive for, your blood type is extremely rare. 

It’s inherited. Like eye color, blood type is passed genetically from your parents. Whether your blood group is type A, B, AB or O is based on the blood types of your mother and father.

* Note: If you have questions about paternity testing or about blood group inheritance, your primary care physician should be able to provide you with an appropriate referral. Testing difficulties can cause exceptions to the above patterns. ABO blood typing is not sufficient to prove or disprove paternity or maternity.

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Blood groups - NHS

There are 4 main blood groups (types of blood) – A, B, AB and O. Your blood group is determined by the genes you inherit from your parents.

Each group can be either RhD positive or RhD negative, which means in total there are 8 blood groups.

Antibodies and antigens

Blood is made up of red blood cells, white blood cells and platelets in a liquid called plasma. Your blood group is identified by antibodies and antigens in the blood.

Antibodies are proteins found in plasma. They're part of your body's natural defences. They recognise foreign substances, such as germs, and alert your immune system, which destroys them.

Antigens are protein molecules found on the surface of red blood cells.

The ABO system

There are 4 main blood groups defined by the ABO system:

  • blood group A – has A antigens on the red blood cells with anti-B antibodies in the plasma
  • blood group B – has B antigens with anti-A antibodies in the plasma
  • blood group O – has no antigens, but both anti-A and anti-B antibodies in the plasma
  • blood group AB – has both A and B antigens, but no antibodies

Blood group O is the most common blood group. Almost half of the UK population (48%) has blood group O.

Receiving blood from the wrong ABO group can be life-threatening. For example, if someone with group B blood is given group A blood, their anti-A antibodies will attack the group A cells.

This is why group A blood must never be given to someone who has group B blood and vice versa.

As group O red blood cells do not have any A or B antigens, it can safely be given to any other group.

The NHS Blood and Transplant (NHSBT) website has more information about the different blood groups.

The Rh system

Red blood cells sometimes have another antigen, a protein known as the RhD antigen. If this is present, your blood group is RhD positive. If it's absent, your blood group is RhD negative.

This means you can be 1 of 8 blood groups:

  • A RhD positive (A+)
  • A RhD negative (A-)
  • B RhD positive (B+)
  • B RhD negative (B-)
  • O RhD positive (O+)
  • O RhD negative (O-)
  • AB RhD positive (AB+)
  • AB RhD negative (AB-)

About 85% of the UK population is RhD positive (36% of the population has O+, the most common type).

In most cases, O RhD negative blood (O-) can safely be given to anyone. It's often used in medical emergencies when the blood type is not immediately known.

It's safe for most recipients because it does not have any A, B or RhD antigens on the surface of the cells, and is compatible with every other ABO and RhD blood group.

The NHS Blood and Transplant (NHSBT) website has more information about the Rh system.

Blood group test

To work out your blood group, your red cells are mixed with different antibody solutions. If, for example, the solution contains anti-B antibodies and you have B antigens on your cells (you're blood group B), it will clump together.

If the blood does not react to any of the anti-A or anti-B antibodies, it's blood group O. A series of tests with different types of antibody can be used to identify your blood group.

If you have a blood transfusion – where blood is taken from one person and given to another – your blood will be tested against a sample of donor cells that contain ABO and RhD antigens. If there's no reaction, donor blood with the same ABO and RhD type can be used.


Pregnant women are always given a blood group test. This is because if the mother is RhD negative but the child has inherited RhD-positive blood from the father, it could cause complications if left untreated.

RhD-negative women of child-bearing age should always only receive RhD-negative blood.

Read more about Rhesus disease.

Giving blood

Most people are able to give blood, but only 1 in 25 people actually do. You can donate blood if you:

  • are fit and healthy
  • weigh at least 50kg (7st 12lb)
  • are 17-66 years old (or 70 if you've given blood before)
  • are over 70 and have given blood in the last 2 years

Read more about who can give blood.

Find your nearest blood donor centre in England and North Wales

You can book an appointment online, or you can call 0300 123 23 23 to book an appointment.

Page last reviewed: 20 April 2020
Next review due: 20 April 2023

About Blood Types - Blood Center

When a doctor talks about your blood type, he usually means two things: your ABO blood type and your Rh (rhesus factor).

A person's blood group is determined by antigens found on his red blood cells. An antigen is a structure on the surface of a cell. If it is foreign to the body, then the human defense system will react to it. Therefore, it is necessary to take into account blood groups when transfusing: the donor's blood group is determined at the Blood Center, and the patient's blood group is determined before transfusion.

AB0 system

The most important is the ABO blood group system, according to which blood is divided into groups A, B, O and AB. It is determined by two antigens located on the surface of red blood cells:

  • group A - only antigen A is present on the surface of erythrocytes
  • group B - only antigen B is present on the surface of erythrocytes
  • group AB - both A and B antigens are present on the surface of erythrocytes
  • group O - on the surface of erythrocytes there is neither antigen A nor antigen B.

If a person has blood type A, B or 0, then in his blood plasma there are also antibodies that destroy those antigens that the person himself does not have. Examples: If you have type A blood, then you cannot be transfused with type B blood, because in this case there are antibodies in your blood that fight against B antigens. If you have blood type 0, then your blood contains antibodies that fight like against antigens A and against antigens B.

If a person has blood type AB, then he does not have such antibodies, so he can be transfused with blood of any group. Therefore, the carrier of the AB blood group can be called universal patient.

A carrier of blood type 0 with a negative Rh factor, in turn, is called a universal donor, , since his red blood cells are suitable for all patients.

Rhesus (Rh ) accessory

Rh factor (Rh) can be positive (+) and negative (-). It depends on the presence of the D antigen on the surface of the red blood cells. If the D antigen is present, the person is considered Rh-positive, and if the D antigen is absent, then the person is Rh-negative.

If a person is Rh negative, contact with Rh positive blood (for example, during pregnancy or blood transfusion) may form antibodies. These antibodies can cause pregnancy problems in an Rh-negative woman if she is carrying an Rh-positive baby.

In addition to the ABO and Rh systems, about thirty more blood group systems have been discovered to date. Clinically, the most important of these are the Kell, Kidd and Duffy systems. According to the Kell system, donor blood is also examined.

How is the blood type determined?

To determine the blood type, it is mixed with a reagent containing known antibodies.

Three drops of blood taken from one person are applied to the base: the anti-A test reagent is added to one drop, the anti-B test reagent to the other, and the anti-D test reagent to the third, i.e. Rh test reagent. If blood clots form in the first drop, i.e. erythrocytes stick together (agglutination), then the person has antigen A. If erythrocytes do not stick together in another drop, therefore the person does not have antigen B; and if agglutination occurs in the third drop, then this indicates a positive Rh factor. In this example, the donor has blood type A and is Rh positive.

The compatibility of the blood type of the donor and the recipient is extremely important, otherwise the recipient may have dangerous reactions to the blood transfusion.

Inheritance of blood groups

A person inherits from his father and from his mother to the same extent. Therefore, the hereditary substance has a double structure: one part from the mother and the other from the father. Speaking about the inheritance of blood groups, it must be borne in mind that:

  • Most of our genes exist in two copies
  • Each parent gives (based on a random selection) one of these copies to their children
  • Genes occur in different versions (alleles)
  • Some versions of the gene are stronger than others

ABO system System Rh
In the AB0 system, antigens are presented in three versions A, B and 0. Given that the hereditary substance consists of two parts, six different combinations can occur:

The stronger part appears, both equally or a combination. In the AB0 system, genes A and B are stronger than 0, which affects the formation of the blood group as follows:

Genes Blood type
A0 A
B0 B
00 0

Example: The mother has a combination of A0 genes in hereditary substance, and her blood type is A (at the same time, she is a carrier of the blood type 0 gene, and there is a possibility that she will pass it on to her child). The father's blood type has the designation 0, and in his hereditary substance there is a combination of genes 00. Accordingly, he can only pass 0 to the child, i.e. absence of antigens. Thus, their child may have an A (A0) or 0 (00) blood type.

In the Rh system, things are somewhat simpler, since there are only two options: the D antigen is either present (Rh positive) or absent (Rh negative). A positive Rh factor dominates a negative one.
Genes Blood type
+/+ ​​ positive
+/- positive
-/- negative

Example: if the mother has Rh positive blood, and there is a hidden negative version, that is, the allele (+/-), and the father has exactly the same combination, and they both pass the negative allele, then two Rh-positive parents can have a child with a negative Rh factor.

How is blood type inherited? - American Medical Clinic

Blood consists of a liquid part - plasma and various blood cells (shaped elements). Plasma contains proteins, minerals. The formed elements of blood are erythrocytes, leukocytes, and platelets. The volume of blood is 6-8% of body weight - about 5 liters. Blood performs a number of important functions: transports oxygen, carbon dioxide and nutrients; distributes heat throughout the body; provides water-salt exchange; delivers hormones and other regulatory substances to various organs; has a protective (immune) function.

Differences between people in terms of blood types are differences in the composition of certain antigens and antibodies.

The existence of blood groups became known only at the beginning of the last century, in 1900-1902, when the Austrian scientist Karl Landsteiner found that when the blood of two different people is mixed, in some cases, red blood cells stick together, in others they do not. This means that not everyone has the same blood, and there are compatible and incompatible blood types. This discovery was of great importance, as it suggested a way to safely transfuse blood: you just need to determine its compatibility.

More than 20 years later, it became known that blood types are inherited and that inheritance occurs in strict accordance with the laws of genetics.

Any hereditary trait is controlled by at least a couple of genes, one of which the child receives from the mother, the other from the father. And in this case, too, parents pass on to the child not a “ready-made” blood type, but one gene responsible for its formation. It depends on the interaction of these genes what kind of blood type the child will have: the same as that of the father, the same as that of the mother, or as a result of a combination of genes, a third variant will arise.

Studying the structure of red blood cells, Landsteiner discovered special substances. He divided them into two categories, A and B, highlighting the third, where he took the cells in which they were not. Later, his students discovered erythrocytes containing A- and B-type markers at the same time.

As a result of research, a system of division into blood groups arose, which was called ABO (read a, b, zero), in which four groups are distinguished. We are still using this system.

  • I (0) - blood group is characterized by the absence of antigens A and B;
  • II (A) - is established in the presence of antigen A;
  • III (B) - is established in the presence of antigens B;
  • IV (AB) - is established in the presence of antigens A and B.

Group I is called zero and is designated as 00, which indicates the presence of two identical genes that determined the sign of the group - one zero was received from the father, the other from the mother.

If a child has I blood type, this means that both the father and mother necessarily have gene 0, but it does not mean at all that they also have group I, since their second genes could be different.

The gene of group II is denoted by the letter A. And if a child receives such a gene from both parents, then, of course, he will have blood group II (AA). But he will have the same group and if he receives gene 0 from one of the parents, and A from the other, since gene 0 has one feature - it cannot manifest itself in the presence of gene A.

The gene of the III blood group is denoted by the letter B. This group is also formed in those who received from their parents two identical BB genes or two different ones - B and 0, because gene 0 does not manifest itself in this combination either.

What happens if a child inherits gene A from one parent and gene B from the other? In relation to each other, they are tolerant, one does not suppress the other, and their combination leads to the appearance of a new sign - blood group IV (AB).

Table of inheritance of the blood type of the child depending on the blood types of the father and mother

Mom + Dad Child's blood group: Options (in %)
I + I I (100%) - - -
I + II I (50%) II (50%) - -
I + III I (50%) - III (50%) -
I + IV - II (50%) III (50%) -
II + II I (25%) II (75%) - -
II + III I (25%) II (25%) III (25%) IV (25%)
II + IV - II (50%) III (25%) IV (25%)
III + III I (25%) - III (75%) -
III + IV - II (25%) III (50%) IV (25%)
IV + IV - II (25%) III (25%) IV (50%)

Rh inheritance

The birth of a child with a negative Rh factor in a family with Rh-positive parents causes deep bewilderment at best, distrust at worst. Reproaches and doubts about the fidelity of the spouse. Oddly enough, there is nothing exceptional in this situation. There is a simple explanation for such a delicate problem.

The Rh factor is an antigen (protein) found on the surface of red blood cells (erythrocytes). It was discovered at 1919 g in the blood of monkeys, and later in humans. Approximately 80-85% of people have it and are accordingly Rh-positive. Those who do not have it (the remaining 15%) are Rh-negative. It is also taken into account in blood transfusion.

These indicators are denoted by the Latin letters Rh with a plus or minus sign, respectively.

The inheritance of blood type and Rh factor occur independently of each other. If both parents are Rh positive, the child will be positive. If both parents are negative. - the child inherits more often - negative. If one of the parents is Rh-positive and the other is Rh-negative, then the probability of the baby's Rh-affiliation is determined by 50% to 50%. There is a possibility of Rhesus inheritance after several generations (the case when the father and mother have a positive Rh, and the born child has a negative Rh). Therefore, when planning a family, parental compatibility studies are mandatory - it is necessary to determine blood groups and blood rhesus. Women with Rh-negative blood are at risk.

A positive Rh factor in a woman and a negative one in a man do not represent any cause for concern. If a woman has Rh-negative blood, and her husband has Rh-positive blood, then Rh conflict may develop during pregnancy, so a woman is recommended to do a blood test for antibodies to the Rh factor before pregnancy. The fact is that if a woman has undergone a surgical operation (including abortion) or a blood transfusion before pregnancy, or if the pregnancy is not the first, then there is a possibility of formation of specific antibodies in her blood. In a Rh-negative woman with a Rh-positive fetus, immune complications (hemolytic disease of the newborn, etc. ) are possible, and especially from the second or third pregnancy. To prevent complications, anti-Rhesus gamma globulin is administered. It is necessary to regularly examine the blood for Rh antibodies in dynamics.

Character inheritance

For centuries, parents only wondered what their child would be like. Today there is an opportunity to look into the beautiful far away. Thanks to ultrasound, you can find out the sex and some features of the anatomy and physiology of the baby.

Genetics allows you to determine the likely color of the eyes and hair, and even the presence of an ear for music in a baby. All these traits are inherited according to the laws of Mendel and are divided into dominant and recessive. The dominant gene is indicated by a capital letter of the Latin alphabet, and in its presence, the recessive gene, as a rule, does not show its properties. The recessive gene is indicated by a capital letter of the Latin alphabet. If, for some trait, an organism contains two identical genes (two recessive or two dominant), then it is called homozygous for this trait. If the organism contains one dominant and one recessive gene, then it is called heterozygous for this trait, and at the same time, those properties of the trait that are encoded by the dominant gene are manifested.

For example,

A is a dominant gene that determines brown eye color
a - recessive gene that determines the blue color of the eyes

Possible variants of the genotype:

AA - homozygous, brown eyes
Aa - heterozygote, brown eyes
aa - homozygous, blue eyes

Brown eyes, hair with small curls, and even the ability to roll the tongue into a tube are dominant signs. Most likely, the child will inherit them.

Unfortunately, the dominant features also include a tendency to early baldness and graying, myopia and a gap between the front teeth.

Gray and blue eyes, straight hair, fair skin, mediocre ear for music are considered recessive. These symptoms are less likely to occur.

Blood type and risk of certain diseases

There is a pattern between blood type and the risk of developing certain diseases (predisposition). Australian scientists have found that people with blood type 0 (I) are much less likely to suffer from schizophrenia. People with type B (III) blood have a higher risk than others of a severe disease of the nervous system - Parkinson's disease. Of course, the blood type itself does not mean that a person will necessarily suffer from a "characteristic" disease for her. There are many factors involved, and blood type is just one of them.

Blood type and human character

Blood type 0 (I). Energetic, sociable, good health, strong will. Striving for leadership. Fussy, ambitious.

Blood group A (II). Diligent and committed. They love harmony and order. Their weakness is stubbornness.

Blood group B (III). Delicate, impressionable, calm. Increased demands on themselves and others. Individualists. Easy to adapt to everything. Powerful and creative personalities.

Blood group AB (IV). Emotions and feelings take precedence over common sense and calculation.

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