Blood Groups - an Overview Content from the guide to life, the universe and everything

Blood Groups - an Overview

3 Conversations

Some blood cells

In the early days of medicine, doctors made the quite reasonable assumption that, if someone was a little short of blood, a quick donation from a friend or relative would solve all their problems. Sometimes this worked, but on many occasions the recipient of the blood became very ill and, more often than not, died. It was not until the beginning of the 20th Century that Karl Landsteiner worked out why this was so and, as a reward for his efforts, was awarded the Nobel Prize in Medicine or Physiology.

Easy as ABO

The most well-known blood groups are those of the ABO system. Everyone in the world can be classified into four categories: A, B, AB and O. Definition of each category depends on the 'antigens' present on red blood cells, and the 'antibodies' present in the blood plasma.

Antibodies and Antigens

Put very simply, antibodies are proteins that make up part of the immune system. Their job is to bind to anything that the body does not recognise, thus labelling it as an invader and directing other parts of the immune system to destroy it. An antigen is any protein or carbohydrate molecule that can be recognised by the immune system. Antigens are classed as 'self' (those that are present in the body normally and do not trigger an immune response) or 'foreign' (those recognised as invaders and attacked).

On The Surface

The antigens of the ABO system are present on the surface of red blood cells. People with blood group A have A-antigens, and people with blood group B have B-antigens. Unsurprisingly, people with blood group AB have both, and people with blood group O have neither.

Which antigens are made depends on which genes you have. There are genes for the A- and B-antigens, and an O gene that produces no antigen. Everyone has two ABO blood group genes, giving six possible combinations:

  • AA - Blood group A
  • AO - Blood group A
  • BB - Blood group B
  • BO - Blood group B
  • AB - Blood group AB
  • OO - Blood group O

As can be seen, having only one A or B gene is enough to produce the antigens creating the blood group, while two copies of the O gene are needed to produce group O. If someone has the A and B genes, they have the AB blood group1.

The O gene is very common, which is why more people have blood group O than any other. The A and B genes are more rare and vary around the world - for example, in Western Europe, the A gene is almost as common as the O gene, while the B gene is very rare. This explains why AB is such a rare blood group.


People with blood group A have A-antigens but no B-antigens. This means that any B-antigens entering the body are recognised as foreign and antibodies are produced. Similarly, a person with group B exposed to A-antigens will produce A-antibodies. Summing all this up:

Blood groupGenesAntigens presentAntibodies present% of UK population2

The important outcome of all this is that if you give B blood to someone of A blood group, then the anti-B antibodies cause all the new blood cells to stick together and block up all the blood vessels. This is very painful, and often fatal.

If you are O blood group, you have no antigens, and therefore your blood can be given to anyone of A, B, AB or O group. Unfortunately, you can only receive O blood. Conversely, if you are AB, you can receive any blood, but yours can only be used in other AB individuals.

These same principles are used in typing someone's blood group. A blood sample is mixed with anti-A and anti-B antibodies. If the blood reacts with either or both of the antibodies, it must contain the relevant antigens. If it reacts with neither, it is group O.

In a Little More Detail...

The A- and B-antigens all begin life as a protein called H, which is found on red blood cells from people with all four blood groups. The A and B genes produce enzymes that modify the H protein into the A- and B-antigens.

Some people cannot make the H protein at all3, and so cannot make ABO antigens, regardless of what ABO genes they have. For transfusion purposes, these people are classed as blood group 'O', as they have no A- or B-antigens.


The other important blood group is known as the rhesus group, after the monkeys in which it was discovered. In principle, the rhesus blood group is simpler than ABO, having only two groups: positive and negative. The same general principles apply as for the ABO system, so rhesus-negative blood can be given to anyone, while rhesus-positive blood can only be given to other rhesus-positive people. However, unlike the ABO system, antibodies to rhesus antigens do not develop until the first time a rhesus-negative person is exposed. Any subsequent exposure to rhesus antigens leads to severe side effects.

In reality, the rhesus blood group is actually quite complicated, consisting of three different genes, various combinations of which produce the positive and negative groups. Because there are more ways of being positive than negative, around 85% of the UK population are rhesus-positive. Again, values vary around the world. People of Afro-Caribbean origin have only a 5% chance of being rhesus-negative, while rhesus-negative individuals are almost unknown in Chinese populations4.

The rhesus blood group is involved in a serious disease of newborn babies - Haemolytic Disease of the Newborn (HDN) - which, fortunately, is now almost completely preventable.

Exchanging Bodily Fluids

When determining the appropriate blood group for transfusions, both the ABO and the rhesus groups must be taken into account. This is why a person's blood group is referred to as 'A-positive', 'AB-negative' and so forth.

Because O-negative blood can, in theory, be given to anyone, regardless of blood group, people with O-negative blood are sometimes referred to as 'universal donors'5. These people are in great demand by blood transfusion services, not least because O-negative is the only blood type that O-negative individuals can receive. Those with AB-positive blood are in the opposite position - their blood is only of use to other AB-positive people, but they can be given any type.

It is important to realise that there is more to giving blood than just your blood group. Medical history is taken into consideration, as well as recent activities that could influence the health of the donated blood. More information on donating blood is published by organisations such as The National Blood Service or Scottish National Blood Transfusion Service in the UK, or the US website

And The Rest

In addition to these main blood groups, red blood cells are coated in a wide variety of other antigens. These are classified into over 20 blood group systems, with names like Duffy, Kell and MNS. These types are less important in transfusions than ABO and rhesus, as they do not induce such strong immune responses. Even so, their existence means that the blood of a patient should always be tested with the blood of a recipient before transfusion, even if the ABO and rhesus groups are a match. Of course, in emergencies, there isn't always time to do this, and medics must rely on the ABO and rhesus groupings, or use O-negative blood6, to give the best chance of a successful transfusion.

1The A and B genes are said to be 'codominant', while the O gene is 'recessive'.2The overall frequencies in the USA are very similar to those in the UK. In other parts of the world, such as the Far East, the frequency of the B group is much higher, giving an approximate distribution of A: 20%, B: 30%, O: 40%, AB: 10%.3This is called the 'Bombay phenotype'.4Of course, providing the blood groups match, it doesn't matter where donor and recipient come from!5Though they are not strictly 'universal', as there are blood groups other than ABO and rhesus that may also be important, as discussed at the end of this Entry.6In men and post-menopausal women, O-positive blood can also be used in an emergency, as there is no danger of HDN developing. Of course, if the person has previously been exposed to rhesus antigens - unlikely though this is - then they will experience a reaction.

Bookmark on your Personal Space

Edited Entry


Infinite Improbability Drive

Infinite Improbability Drive

Read a random Edited Entry

Categorised In:

Write an Entry

"The Hitchhiker's Guide to the Galaxy is a wholly remarkable book. It has been compiled and recompiled many times and under many different editorships. It contains contributions from countless numbers of travellers and researchers."

Write an entry
Read more