From the moment we are born, we are given an invisible label that will stay with us for the rest of our lives: our blood type. It tells us whether we are type A, B, AB or O, and whether we are Rh positive or negative. Yet, while this information is crucial to our health, we rarely stop to think about what it really means.
Why is it important to know our blood type? What role does it play when we donate or receive blood? And most importantly, how can this information make a difference in emergency situations?
Blood types not only define what type of blood we have, but are also key in medical situations such as transfusions, surgeries, transplants or even pregnancies. The ABO system and the Rh factor are essential to avoid complications that could be life-threatening.
We invite you to discover what blood groups are, how they work and why they are essential in the blood donation process.
Did you know that a single donation can save up to three lives? Join us and learn how your blood can be the greatest gift someone can receive.
Blood type and Rh factor
Blood groups are classifications based on the presence or absence of certain antigens on the surface of red blood cells. Antigens are the marks that the body uses to recognize its own cells and distinguish them from invading or foreign cells, so if it receives invading cells, the body detects them through these antigens and triggers an immune response to eliminate those cells. This is the reason why we cannot receive incompatible blood or organs and it is also a main point of study in the field of autoimmune diseases, which will be dedicated to an article of their own.
The most well-known and widely used system for classifying blood types is the ABO system, which divides blood into four main groups: A, B, AB and O.
ABO system
The ABO blood group system was discovered by Austrian physician and scientist Karl Landsteiner in 1901. Landsteiner made this discovery when he observed that when he mixed blood samples from different people, in some cases the blood cells clumped together (agglutinated), while in other cases no reaction occurred. From these experiments, he identified three different blood types, which he named A, B, and C (the latter later renamed O). Soon after, a group of Landsteiner's students identified the fourth type, AB.
Landsteiner's discovery revolutionized medicine, as prior to his work, blood transfusions were extremely dangerous and often resulted in death due to immune reactions caused by incompatibility between different blood types. Thanks to his work, blood transfusions became much safer, as doctors were able to ensure that donors and recipients had compatible blood types.
For this achievement, Karl Landsteiner was awarded the Nobel Prize in Physiology or Medicine in 1930, in recognition of his discovery of the ABO system, which remains one of the fundamental pillars of modern medicine.
- Group A: It has the A antigen on the surface of the red blood cells and the anti-B antibody in the plasma.
- Group B: It has the B antigen on the surface of the red blood cells and the anti-A antibody in the plasma.
- Group AB: Has both antigens, A and B, on the surface of red blood cells and has no anti-A or anti-B antibodies. This is the universal recipient, as it can receive blood from all groups.
- Type O: There are no A or B antigens on the surface of the red blood cells, but there are anti-A and anti-B antibodies in the plasma. Type O negative is the universal donor, as their blood can be used in people of any type.
Rh factor
The Rh factor is a protein known as the D antigen that can be present on red blood cells. If you have it, you are Rh positive (Rh+) , and if you don't have it, you are Rh negative (Rh-) . Rh plays an important role in compatibility for blood transfusions and in pregnancies. For example, if an Rh-negative mother is pregnant with an Rh-positive baby, her immune system may react against the fetus's red blood cells, which can cause complications if not treated properly.
By combining the ABO system with the Rh factor, we obtain the eight main blood types:
A+ | A- | B+ | B- | AB+ | AB- | O+ | O-
Each of these blood types has its own characteristics regarding who can donate or receive blood. The most common are O+ and A+, while the rarest are AB- and B-.
Blood group inheritance
Blood type is inherited genetically, through the genes we receive from our parents. Each person has two alleles (versions of a gene) for the ABO blood group, one inherited from the father and one from the mother. These alleles can be A, B or O, and the combination of these will determine a person's blood type. Similarly, the Rh factor (+ or -) is also inherited through the genes of the parents.
Inheritance of the ABO system
The ABO system follows a codominant pattern of inheritance . This means that the A and B alleles are dominant, while the O allele is recessive. The possible combinations of alleles we inherit from our parents determine our blood type.
The allele combinations for the ABO system are as follows:
- A + A = A
- A + O = A
- B + B = B
- B + O = B
- A + B = AB
- O + O = O
Examples of blood group inheritance:
- If both parents have blood type A (with AO or AA alleles), their children may have type A or O, but not B or AB.
- If one parent has type A (AO) and the other has type B (BO), the child could have any of four blood types: A, B, AB, or O.
- If both parents have group O (OO), their children can only have group O, since the O allele is recessive and there are no dominant alleles A or B present.
Inheritance of the Rh factor
The Rh factor is inherited independently of the ABO system and follows a dominant-recessive inheritance pattern. The Rh+ allele is dominant, and the Rh- allele is recessive.
- If a person inherits at least one Rh+ allele from one of his or her parents, he or she will be Rh positive .
- If a person inherits two Rh- alleles (one from each parent), he or she will be Rh negative .
Examples of Rh factor inheritance:
- If both parents are Rh positive but carry one Rh negative allele (heterozygous), it is possible for them to have an Rh negative child.
- If one parent is Rh positive (with two Rh+ alleles) and the other is Rh negative (with two Rh- alleles), all their children will be Rh positive.
- If both parents are Rh negative, all children will be Rh negative.
Donation compatibility:
A+ -> You can donate to A+ and AB+
A- -> You can donate to A+, A-, AB+ and AB-
B+ -> You can donate to B+ and AB+
B- -> You can donate to B+, B-, AB+ and AB-
AB+ -> You can donate to AB+
AB- -> You can donate to AB+ and AB-
O+ -> You can donate to A+, B+, AB+ and O+
O- -> You can donate to all A+, A-, B+, B-, AB+, AB-, O+ and O-
Why is an Rh+ only compatible with another Rh+ while the negative is compatible with Rh+ and Rh-?
Rh factor compatibility in blood transfusions is related to the presence or absence of the D antigen on the surface of red blood cells. This antigen is what distinguishes people who are Rh positive from those who are Rh negative .
Why can Rh+ only receive from Rh+?
- People with Rh+ have the D antigen on their red blood cells. Because they already have this antigen, their immune system recognizes it as their own and does not generate antibodies against it.
- However, if an Rh+ person receives blood from an Rh- donor, this usually does not cause problems, because Rh- blood lacks the D antigen, and the recipient's body does not perceive it as foreign. For this reason, in theory, Rh+ people can receive both Rh+ and Rh- blood.
Why can Rh- only receive Rh- blood?
People who are Rh negative do not have the D antigen on their red blood cells, meaning their immune system has never been exposed to it. If an Rh- person receives blood from someone who is Rh+ , the D antigen in the Rh+ blood will be recognized as foreign. In response, the Rh- person's immune system will produce antibodies against the D antigen.
This attack by the immune system can cause a hemolytic reaction, in which transfused red blood cells are destroyed, which can lead to serious complications, such as severe anemia, kidney failure, or even death. For this reason, people with Rh- should receive blood only from Rh-negative donors, to avoid this type of reaction.
Why is Rh- compatible with Rh+ and Rh- when donating?
In terms of blood donation , an Rh-negative person can donate to someone who is Rh-positive because Rh- blood does not contain the D antigen, making it “neutral blood” in terms of the Rh factor. Since the Rh+ recipient already has the D antigen, there will be no immune reaction to receiving Rh- blood, as there is no new antigen for their immune system to attack.
Curiosities about blood, our genetic heritage and donations
Finally, some interesting facts about these aspects that we hope will spark your interest:
- Globally, the most common blood group is O+ , which is present in about 40% of the population. The rarest is AB- , which is found in less than 1% of people. However, the distribution of blood groups varies by region. For example, in Asia, blood group B is more common than in Europe or America.
- People with O-negative blood are considered universal donors . Their blood can be transfused to people of any blood type, as they do not have the A, B or Rh antigens on their red blood cells, which minimizes the risk of an immune reaction.
- People with AB positive (AB+) blood type are universal recipients . They can receive blood from any type (A, B, AB and O) without risk of their body rejecting the red blood cells, because they do not produce antibodies against the A, B or Rh antigens.
- In countries like Japan and South Korea, it is believed that blood type can influence a person's personality. For example, people with type A are said to be organized and calm, while those with type O are more outgoing and confident. Although there is no scientific basis for these beliefs, the idea is deeply rooted in popular culture.
- Some studies have found a link between blood type and predisposition to certain diseases. For example, people with type O have a lower risk of developing heart disease, but a higher risk of stomach ulcers. People with type A and type AB have a higher risk of developing gastric cancer.
- By donating just one unit of blood (approximately 450 ml), up to three lives can be saved. This is because donated blood is separated into different components: red blood cells, plasma and platelets, which can be used individually for different patients.
- Plasma from people with type AB is special because it is compatible with any other blood type, making it a “universal” plasma donor. This plasma is known as “golden plasma” due to its importance and usefulness in transfusions.
- As with blood transfusions, blood group compatibility is crucial for organ transplants. A donor's organ can only be received by someone whose blood group is compatible, otherwise the recipient risks rejecting the organ.
- Although not common, attempts have been made to transplant organs from animals (such as pigs) into humans in desperate situations. These are known as xenotransplants, and blood compatibility is one of the main obstacles to the success of these procedures.
- Blood inheritance follows Mendelian patterns: The way we inherit blood types follows the laws of genetics discovered by Gregor Mendel. The A and B alleles are dominant, while the O allele is recessive. This means that two parents with both A and B blood types could have a child with any of the four blood types (A, B, AB, or O).
- Two Rh-positive parents can have an Rh-negative child: This occurs when both parents are heterozygous for the Rh factor (they have one Rh+ allele and one Rh- allele). If both parents transmit the Rh- allele to the child, the child will be Rh-negative, even though both parents are Rh-positive.
- Although blood types are not as precise as DNA in determining paternity, they can provide useful clues. For example, if a father is type O and the mother is type AB, they could not have a child of type A or B, since the alleles would not match that combination.
Bombay blood group
The Bombay blood group is a rare variation of the ABO system, discovered in Bombay (now Mumbai) in 1952. People with this blood group completely lack the H antigen, which is a precursor needed to form the A and B antigens on red blood cells. As a result, these people appear to have type O blood in clinical tests, since they do not have either the A or B antigen, but in fact they have a mutation that prevents the formation of the H antigen.
People with this blood group cannot receive transfusions of blood types A, B, AB or even O, as they will produce antibodies against the H antigen present in these bloods.
They can only receive blood from another individual with the Bombay blood group, which can complicate donor availability since Bombay blood group is extremely rare.
It is estimated that only about 0.0004% of the world's population has it. That is, approximately 1 in every 250,000 people has this blood type.
In some regions of India, where it was discovered, the prevalence is slightly higher, affecting approximately 1 in 10,000 people .
Did you know that your blood type can change?
Although it is very unlikely and is associated with different diseases. In some cases the change is temporary during the course of the disease and in a few circumstances this change is permanent. The most common case of permanent blood group change is in bone marrow transplants .
In a bone marrow transplant , the recipient's blood type may change if the donor has a different blood type. This happens because bone marrow is the tissue responsible for producing blood cells, including red blood cells, which carry the antigens that determine blood type. Once the transplanted marrow begins to function in the recipient's body, it starts producing red blood cells with the donor's blood type. However, for the transplant to be successful, it is crucial to avoid rejection, not only of the organ itself, but also of the "new blood" that is produced.
HLA compatibility
The compatibility between the recipient and the donor is not only based on blood group, but also on human leukocyte antigens (HLA) . HLAs are proteins found on the surface of cells and play a key role in the immune system, helping the body to differentiate between its own cells and foreign ones. Before the transplant, an HLA compatibility test is performed to ensure that the recipient's immune system does not recognize the new marrow as something “foreign” and does not attack it.
- The more HLA compatible the donor and recipient are, the lower the risk of rejection.
- Often, a family donor (such as a sibling) who is a close HLA match is sought.
Recipient preparation: chemotherapy and radiotherapy
Before a bone marrow transplant, the recipient undergoes a conditioning process, which usually includes chemotherapy and sometimes radiotherapy. This treatment has two main objectives:
- Removing the recipient's original bone marrow: In order for the donor's marrow to establish itself and produce new blood cells, the recipient's diseased or damaged marrow must be removed.
- Suppressing the recipient's immune system: This reduces the chance that the recipient's immune system will attack the new donor marrow, which could cause rejection.
Administration of immunosuppressants
After the transplant, the recipient is given immunosuppressive drugs to prevent rejection. These drugs suppress the activity of the recipient's immune system, reducing the chances that the body will attack the new bone marrow and the blood cells it produces. The most common are cyclosporine and methotrexate. These drugs must be carefully managed, as lowering the immune response too much can make the recipient more vulnerable to infections, with fatal consequences.
Control of evolution
Blood group change does not occur immediately after the transplant. In the weeks or months that follow, the recipient's blood cells gradually regenerate, replacing old blood cells with new ones from the donor's marrow. During this time, the recipient's immune system must be suppressed so that the new cells can coexist peacefully without triggering an immune response that attacks them.
Doctors regularly monitor the recipient's blood cell levels to ensure that the transplanted marrow is functioning properly and producing healthy blood cells. It is also monitored for signs of rejection or complications, and immunosuppressant doses are adjusted as needed.
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