The ABC of ABO (A basic overview to blood types and transfusion)
By Trevor and Ann
What’s your blood group?
Unless you have given a blood donation or had a blood transfusion many of us would not be able to answer this question. Most people have heard of blood groups such as A negative, O positive etc, but how many of us actually understand what they mean and the differences between each group?
For example do you know?
Which is the universal blood donor group for the ABO system?
If one of your parents is blood group A and the other is group B, which of the following blood groups would you likely be?
e) Any of the above
Blood is made up of four major components: red blood cells, which carry oxygen around the body; white blood cells, the body’s defence mechanism; platelets, which help form blood clots; and plasma, which is the liquid part of the blood.
A blood group (also called a blood type) is a classification of blood based on the presence or absence of inherited antigenic substances on the surface of red blood cells (RBCs). These antigens may be proteins, carbohydrates, glycoproteins or glycolipids, depending on the blood grouping system. The International Society of Blood Transfusion currently recognises 30 major blood group systems for humans. Each person has a unique spectrum of blood groups with the exception of identical twins or triplets whose blood groups are exactly the same. The most important blood group systems for humans are the ones we call the ABO system, and the Rh system (Rh).
The ABO System
All human beings are either of blood group O, A, B or AB. A person’s ABO blood group is a direct result of two genes, one inherited from the biological mother and one inherited form the biological father. In the simplest terms this gene may be said to come in three different forms (that are called alleles). These alleles are also named A, B and O, because each is responsible for the production of its namesake glycoprotein antigen expressed on the RBCs. It is the combination of alleles that you inherit from your parents that determines which antigens you have on your red cells and thereby your ABO blood type.
The alleles that code for Group A and Group B are said to be dominant over the allele that codes for group O. Therefore only people with two copies of the O allele – one from each parent will be blood group O. Individuals with AA or AO have type A blood and individuals with BB or BO have type B. AB people express both A and B antigens on their red cells in a co-dominance relationship and are thus group AB. With it so far?
The ABO system also involves the presence of antibodies in the plasma So, for example, blood group A has antibodies to the group B antigen in their plasma, B has antibodies to group A, group AB has no antibodies and group O has both.
Understanding the difference between antibodies and antigens is essential to understanding the ABO system. Antibodies are substances produced by the body when it is ‘invaded’ and act to protect it from a foreign substance or antigen. Examples would include infectious agents such as bacteria and viruses and inducing antibody production in response to a pathogenic bug is the principle of how vaccination works.
Typically, the body does not produce antibodies until after it has been introduced to a new substance. However where the ABO blood group system is concerned we develop the antibodies to other blood groups without our immune systems being stimulated by exposure to ‘foreign’ blood. Rather anti-A antibodies and anti-B antibodies are usually produced in the first year of life or even in the womb by sensitisation to environmental substances such as food, bacteria and viruses.
The importance of the ABO antigen antibody system comes into play during the process of blood transfusion. If, at some time in your life, you require a blood transfusion your ABO blood group will decide whose blood you can be given. Thus for example group A blood is dangerous if given to a group B person because the anti- A in the plasma of the group B person will react with the transfused group A red cells. This reaction (known as a transfusion reaction) can be severe enough to cause death.
Blood Group O naturally produce antibodies in their blood to both Group A and B cells and so can only be transfused with blood from another Group O person but people who are Blood Group AB do not produce any blood group antibodies so can be transfused with blood from any donor (A, B, AB or O) as they should not have an antigen-antibody reaction with any of these cells. These individuals are known as universal recipients, whereas individuals with Blood Group O are known as universal donors as their red blood cells can be given to anyone no matter what their blood group is. )
The Rh System
Another typing system that helps in describing a person’s blood type is the Rh blood system and the D antigen is the most important in this system as it is very likely to cause an immune reaction. The presence of D antigen on the surface of your blood cells means you are Rh positive (RhD positive) and its absence implies you are RhD negative. Thus combining this system with ABO typing can give a combination such as O RhD positive or AB RhD negative and so on.
If you have RhD negative blood, your body may form antibodies against Rh positive blood and destroy it. In order for this to happen, you must first be exposed to RhD positive blood. This could be through a blood transfusion or, if you are a pregnant woman, by carrying a RhD positive foetus. (Compare this to the naturally occurring antibodies in the ABO system). This can be a problem if you have antibodies against RhD positive blood and are pregnant with a RhD positive foetus.
Rh D antigen in pregnancy
A situation can arise that a mother can be RhD negative but her foetus can be RhD positive (inherited from the father). If the mother is exposed to the baby’s blood during the pregnancy she can develop Anti-D antibodies and these are capable of passing across the placenta and attacking the babies RhD positive red cells resulting in what is called Haemolytic Disease of the New Born (HDN). This condition however can be prevented and treated but this is very expensive and not always available in developing countries.
Before blood typing systems were discovered and understood, blood transfusions often resulted in the death of the patient. It was Karl Landsteiner in 1901, an Austrian physician, and the most important individual in the field of blood transfusion that documented the first three human blood groups A, B and O. In 1902 the AB blood group was described by Descastrello and Sturli. In 1907 Hektoen an American Pathologist of Norwegien descent suggested that the safety of transfusion might be improved by cross-matching blood between donors and patients to exclude incompatible mixtures. Between 1939 and 1940, the work of Levine, Stetson, Landsteiner and Wiener led to the discovery of the Rh blood grouping system, the second most important blood group system. Once reliable testing for this system had been established, transfusion reactions became rare.
Distribution of blood groups in the population
The percentage of each of the four ABO blood groups present in the population varies in different parts of the world. In Malawi, according to a recent study by Dr. B M’baya and colleagues at the Malawi Blood Transfusion Service, approximately 50% of Malawians are group O, 26% group A, 20% group B and 4% group AB. 97% of Malawians are RhD positive. Compare this with the distribution of ABO blood groups in the English population for example where 47% are group O, 41% A, 9% B and 3% AB. 85% are RhD positive. There are certain ethnic groups in South American where almost 100% of the population are group O. The RhD antigens are also very common in Asian communities where up to 99% of the Taiwanese for example are RhD positive.
The purpose of blood groups in evolutionary terms is not known however, specific ABO and other blood types are thought to be linked with increased or decreased susceptibility to particular diseases.
Animals have blood groups which are not the same as human blood groups but are species specific. So blood from one species is not effective when transfused to another species and indeed could be harmful or even lethal. However ABO blood types are present in some apes such as chimpanzees, bonobos, and gorillas.
One of the benefits of donating blood is that you get to know your blood group and it will help you know if you or your spouse has a risk of haemolytic disease of the new born.
There are numerous popular conjectures surrounding ABO blood groups. These beliefs have existed since the ABO blood groups were identified and can be founded in different cultures throughout the world. For example during the 1930s connecting blood groups to personality types became popular in Japan and other areas of the world.The popularity of Peter J.D’Adamo’s book, ‘Eat Right For Your Blood Type’ suggests that these ideas persist. This book claims that ABO blood type determines one’s optimal diet.
Some of the myths include the idea that group A people can suffer from severe hangovers, group O is associated with perfect teeth, and those with blood group A2 (a subgroup of blood group A) have the highest IQs. Scientific evidence in support of these ideas is nonexistent.