Finding out your blood type and understanding what it reveals about health

A, B, AB or O, plus Rhesus-positive or Rhesus-negative: your blood group is one of those pieces of medical information that many people only become interested in when it is actually needed. Perhaps you are about to donate blood, a pregnancy raises questions or a health check sparks your curiosity. Others wonder whether their blood group says anything about their personality, diet or life expectancy.

In fact, behind these few letters lies a complex biological system. Your blood group influences which units of donated blood can be used for a transfusion and plays an important role in certain medical situations. Associations between blood groups and individual diseases are also being researched. However, by no means every statistical association is medically significant.

Blood group is a genetically shaped characteristic, but not a comprehensive health profile. People who want to find out their blood group have several options. Which one is appropriate depends on whether they are seeking an initial indication, a medically reliable determination or an answer to a specific medical question.

Contents

1. What actually determines blood group
2. How to find out your blood group
3. Can blood group be inferred from your parents?
4. What blood groups reveal about health
5. Blood group and personality: what is scientifically established?
6. Eating according to blood group: useful or a myth?
7. Why blood group matters in medicine
8. What your blood group means for healthy ageing
9. References

What actually determines blood group

Blood looks similar in everyone. Under the microscope, however, there are differences on the surface of red blood cells, known as erythrocytes. Certain structures found there are called blood group antigens. The immune system can recognise these structures and, where applicable, form antibodies against foreign antigens.

Which antigens are present is largely determined by genetic make-up. Numerous blood group systems are known in medicine. The most familiar are the ABO system and the Rhesus system, because they play a particular role in blood transfusions and pregnancy.

The ABO system: A, B, AB or O

The ABO system distinguishes four basic blood groups:

The table describes the usual characteristics of the ABO system. In medical practice, rare subgroups and particular combinations may require additional investigations.

The differences arise from certain sugar structures on the surface of red blood cells. In blood group A, an enzyme modifies a particular structure to produce the A antigen. In blood group B, a different enzyme leads to the formation of the B antigen. In blood group AB, both variants are present. In blood group O, the corresponding enzyme activity is absent, so neither the A nor the B antigen is formed.

These small molecular differences have major medical significance. If red blood cells with incompatible ABO characteristics enter the circulation, existing antibodies can trigger an immune response. In an adverse case, the transfused cells are destroyed.

What does the Rhesus factor mean?

Alongside the ABO blood group, the Rhesus factor is often given. In everyday usage, this usually refers to the Rhesus D characteristic.

If the D antigen is present on red blood cells, the blood group is described as Rh-positive. If it is absent, it is described as Rh-negative. This produces combinations such as A positive, O negative or AB positive.

However, the Rhesus factor is only one part of the much more complex Rhesus blood group system. Alongside D, there are other antigens, including C, c, E and e. These can also be relevant to blood compatibility.

The full blood group designation is therefore not simply A or O. Additional characteristics must be considered in certain medical situations. Which of these are investigated depends on the reason for testing.

How to find out your blood group

People who want to know their blood group do not necessarily need an extensive health check. There are several options, which differ in terms of effort, significance and medical use.

1. Check previous medical records

The simplest first step is to look at existing records. Your blood group may already have been determined during a previous operation, pregnancy or certain medical investigations.

Records from an earlier blood donation may also contain blood group information. What matters is that the information is actually documented and clearly attributable to the person concerned.

An entry in old records can provide a useful indication. For a planned transfusion, however, medical facilities do not simply rely on a verbal statement or a blood group card brought in by the patient. The necessary investigations and compatibility checks are carried out according to the applicable medical standards.

2. Have your blood group determined by a doctor

Blood group can be determined through a blood test at a medical practice or laboratory. A blood sample is examined using suitable test reagents.

In traditional serological testing, red blood cells are brought into contact with specific antibodies. If the cells clump together on contact with anti-A, for example, this indicates the presence of the A antigen. Further reactions help to classify the blood group.

For a complete ABO determination, the blood plasma is usually also examined. The laboratory checks whether the expected antibodies against the missing antigens are present. Combining both testing steps increases the reliability of the classification.

Depending on the question being investigated, the Rhesus factor is also determined. Particular findings may require further investigations.

Whether the costs are covered depends on the medical reason. Blood group testing without a specific medical indication may be a self-funded service. It therefore makes sense to ask the practice beforehand.

3. Find out your blood group when donating blood

Donating blood can also be a way to find out your blood group. Blood donation services determine relevant blood group characteristics as part of the prescribed investigations.

However, nobody should donate blood solely to find out their blood group. Donation requires the person to meet the relevant criteria and actually want to donate. Information on whether and how blood group details are communicated is available from the relevant blood donation service.

It is also important to know that blood group is not determined or documented in the same way for every form of blood donation. People who need reliable medical information should therefore not rely solely on an assumed later notification.

4. Self-tests for home use

Rapid blood group tests are commercially available and use a small blood sample from the fingertip. They are also based on immunological reactions in which specific antibodies react with blood group antigens.

These tests can provide an initial indication. However, their significance depends on correct use, product quality and clear interpretation of the result. Errors in collecting the sample or reading the result can lead to incorrect conclusions.

A self-test does not replace medical blood group testing when the result is needed for treatment. In particular, before a transfusion, the compatibility of blood products must never be assessed using a home test or a self-documented blood group.

Which method is right?

People who simply want to find out their blood group out of curiosity can first check existing records. If the information is needed for medical reasons, professional testing is the right approach.

Can blood group be inferred from your parents?

Blood group is inherited. Under certain conditions, the possibilities can therefore be narrowed down using the parents' blood groups. However, it cannot always be determined conclusively from them.

In the ABO system, a child receives one gene variant from the mother and one from the father. The A and B variants are dominant over the O variant. A and B are codominant with one another, meaning both characteristics can be expressed simultaneously.

This produces various combinations. A person with blood group A, for example, may have the gene variants AA or AO. For blood group B, BB or BO are possible. Blood group AB corresponds to the AB combination, while blood group O normally results from two O variants.

These differences explain why parents with the same expressed blood group do not necessarily have children with the same blood group.

For example, if both parents have blood group A, their children may have blood group A or O, depending on the genetic combination. If both parents are genetically AO, both possibilities exist. By contrast, where parents have blood group AB, blood group O is not expected under the usual inheritance conditions.

Actual inheritance is more complex than a simple table might initially suggest. Rare genetic variants and particular features of blood group testing can depart from the usual rules.

Can the Rhesus factor also be inferred?

The Rhesus factor is also genetically determined. However, its inheritance cannot always be fully reconstructed from the designation positive or negative alone.

If both parents are Rh-negative, an Rh-positive child is not expected under the usual inheritance conditions. However, two Rh-positive parents can also have an Rh-negative child if both pass on the corresponding genetic variant.

The parents' blood groups provide clues, but do not replace a test. If your own blood group is medically important, it should be determined directly rather than relying on calculations or information within the family.

What blood groups reveal about health

Blood groups are of interest beyond transfusions. Scientific studies have long investigated whether certain blood groups are associated with diseases or other health characteristics.

Such associations are biologically plausible. Blood group antigens are not found exclusively on red blood cells. Depending on the blood group system, corresponding structures also occur on other cells or in body fluids. Among other things, they can influence how certain pathogens bind to cells or how individual components of the clotting system function.

Nevertheless, a statistical association is not automatically proof that blood group causes a disease. Nor does it mean that personal disease risk can be reliably predicted from blood group alone.

Blood group and cardiovascular health

One area of research concerns the association between blood groups and cardiovascular disease. Observational studies suggest that people with a non-O blood group may, on average, have different levels of certain clotting factors from people with blood group O.

These include von Willebrand factor and clotting factor VIII. Both are involved in blood clotting. Their concentrations may be associated with the risk of venous thrombosis.

A review from 2024 assessed systematic reviews and meta-analyses on blood groups and various health outcomes. It examined a total of 270 associations between ABO or Rhesus blood groups and possible health consequences. The authors judged only one association to be supported by convincing evidence. Many other results were less robust or required further research.

This shows how cautiously such findings must be interpreted. Even when statistical differences can be observed for certain blood groups, individual risk depends on numerous other factors, including age, pre-existing conditions, exercise, smoking and genetic predisposition.

Blood group alone does not allow a reliable prediction of whether someone will develop cardiovascular disease. Established factors such as blood pressure, blood lipids and blood sugar are much more informative for medical risk assessment.

Blood groups and infectious diseases

Research also examines whether blood group antigens play a role in infectious diseases. Some pathogens can use certain surface structures to interact with cells. In theory and in individual studies, this can explain why certain blood groups are observed at different frequencies in some infections.

However, the results vary depending on the pathogen, population studied and study design. An association observed for one infection cannot automatically be applied to other diseases.

Moreover, blood group effects are often just one factor among many. Age, immune status, pre-existing conditions, exposure to a pathogen and other genetic characteristics may play a much greater role.

Blood group therefore cannot reliably indicate either how susceptible a person is to infections or how severe a disease will be.

Blood group and cancer

Possible associations between blood groups and certain cancers have also been investigated. Some studies report differences in the frequency of individual tumour types between different blood groups.

These observations are initially statistical associations. They neither prove a direct causal link nor allow reliable individual predictions. Results also differ by disease and population.

For personal preventive care, blood group is therefore no substitute for established recommendations. Age, family history, known genetic risks and other medical factors are much more important in assessing individual risk.

What does this mean for your own preventive healthcare?

Research into blood groups can help improve our understanding of biological relationships. However, it does not automatically lead to new recommendations for everyday life.

Knowing your blood group therefore does not mean you fundamentally need to eat differently, train differently or plan different screening examinations. Nor is a supposedly particularly favourable blood group a reason to neglect other risk factors.

A statistical association is not a personal diagnosis. Blood groups can be relevant in certain medical contexts, but should not be used in isolation to assess your own health.

Blood group and personality: what is scientifically established?

In some countries, particularly Japan, the idea that blood group reflects certain character traits is widespread. In such beliefs, people with blood group A are considered especially orderly, for example, while blood group B is often associated with independence and spontaneity.

These attributions are culturally familiar, but are not supported by convincing scientific evidence. A study by Kengo Nawata analysed large population samples from Japan and the USA, totalling more than 10,000 participants. For 65 of the 68 characteristics examined, there were no statistically significant differences between blood groups. Overall, blood group explained less than 0.3 per cent of the differences in the personality traits examined.

This strongly argues against the assumption that character can be meaningfully described on the basis of blood group.

Why do such descriptions sometimes seem accurate nonetheless? General personality descriptions often leave plenty of room for interpretation. People also tend to remember examples that fit an existing belief rather than contradictory experiences.

Your personality is shaped by numerous factors, including genetic influences, development, upbringing, social environment and personal experiences. Blood group is not a scientifically established tool for determining these characteristics.

People interested in personality traits should therefore not use blood groups as a basis for self-diagnosis or assessments of other people.

Eating according to blood group: useful or a myth?

The idea of the blood group diet assumes that people should favour or avoid different foods depending on their blood group. Depending on the version of the concept, certain diets are recommended for blood group A, for example, while people with blood group O supposedly need more animal-based foods.

The explanations often refer to evolution, the emergence of different blood groups or possible interactions between food components and blood group antigens. These explanations sound biologically plausible at first, but are not sufficient to justify specific dietary recommendations.

For the blood group diet, there is no convincing scientific evidence that a diet specifically tailored to the ABO blood group achieves better health outcomes than a balanced diet without this restriction.

People may indeed feel better after such a dietary change. However, that does not mean blood group is responsible. Someone who eats more vegetables, pulses and wholegrain products, for example, or reduces highly processed foods, is also changing the overall quality of their diet.

What matters, therefore, is which foods are actually eaten, how balanced the nutrient intake is and whether the diet meets individual needs.

What really matters in a balanced diet

A diet aimed at supporting health should follow scientifically well-founded recommendations rather than blood group. These include a varied selection of plant-based foods, sufficient fibre and an intake of important nutrients that meets requirements.

Personal factors also play a role. Age, physical activity, diseases, intolerances and individual needs can influence which diet makes sense in everyday life.

There is no convincing evidence for eating according to blood group that would justify it as a general recommendation. People who want to improve their diet are therefore better advised to pay attention to the actual composition and quality of their food.

Why blood group matters in medicine

Although blood group tells us only a limited amount about general health, it is indispensable in certain medical situations. It is particularly important in blood transfusions and pregnancy.

Blood transfusions and compatibility

In a blood transfusion, it is necessary to check whether the blood products are compatible with the recipient. If the ABO characteristics are incompatible, the recipient's antibodies can attack the transfused red blood cells. This can trigger severe, potentially life-threatening reactions.

Before a planned transfusion, blood group is therefore determined and further compatibility investigations are carried out. Depending on the situation, these include antibody screening and serological compatibility testing, also called crossmatching.

ABO group and Rhesus factor are important characteristics, but not the only ones. Other blood group antigens can also be relevant, particularly if someone has previously received blood transfusions or formed certain antibodies.

The frequently used terms ‘universal donor’ and ‘universal recipient’ refer to simplified combinations within the ABO and Rhesus D systems. They do not mean that every unit of donated blood is suitable for every person regardless of other characteristics. In medical care, the selection is therefore not made solely on the basis of these shorthand terms.

Pregnancy and Rhesus factor

The Rhesus factor can also be relevant during pregnancy. If a pregnant woman is Rh-negative and her unborn child is Rh-positive, contact between the child's red blood cells and the mother's immune system can occur under certain circumstances.

This can cause the mother to form antibodies against the D antigen. If these antibodies later pass through the placenta to the child, they can attack the child's red blood cells. This risk must be considered particularly where such sensitisation has occurred.

Antenatal care and appropriate medical measures now allow this risk to be addressed specifically. These include determining the Rhesus factor, testing for relevant antibodies and, where indicated, anti-D prophylaxis.

The precise approach depends on the medical situation and applicable recommendations. People who are pregnant or planning a pregnancy can discuss the significance of their blood group during gynaecological care.

What your blood group means for healthy ageing

The subject of longevity concerns maintaining health and capability over many years of life. Numerous factors interact here: exercise, diet, sleep, social relationships, genetic characteristics and early detection of relevant diseases.

Although blood group is a genetically shaped characteristic, it is not a general marker of biological age. It does not show how fit someone is, how quickly a person is ageing or how long they will live.

Even though research examines associations between certain blood groups and individual diseases, this does not allow a comprehensive statement about life expectancy. Many observed links are limited, inconsistent or not yet sufficiently understood.

Other factors therefore remain central to long-term health. These include controlling blood pressure and blood lipids, regular exercise, not smoking, a balanced diet and making use of appropriate preventive care.

Knowing your blood group can nonetheless be practical. It is important information for certain medical situations and helps complete your medical history. However, it should not serve as a basis for unproven dietary concepts or far-reaching health predictions.

People who want to find out their blood group can first check existing medical records or ask a medical practice about testing. Medical decisions require a reliably established result.

Blood group reveals something about certain biological characteristics, but by no means everything about a person.
For a healthy life, it is one small piece of the puzzle. What remains crucial is the overall picture of genetic characteristics, lifestyle and medically relevant risk factors.

References

1. Dean, L. (2005). Blood Groups and Red Cell Antigens. National Center for Biotechnology Information, Bethesda, MD.
2. Avent, N. D., Reid, M. E. (2000). The Rh blood group system: a review. Blood, 95(2), 375–387.
3. Liu, F.-H. et al. (2024). ABO and Rhesus blood groups and multiple health outcomes: an umbrella review of systematic reviews with meta-analyses of observational studies. BMC Medicine, 22, 206.
4. Nawata, K. (2014). No relationship between blood type and personality: evidence from large-scale surveys in Japan and the US. Japanese Journal of Psychology, 85(2), 148–156.
5. Rogers, M., Glendon, A. I. (2003). Blood type and personality. Personality and Individual Differences, 34(7), 1099–1112.