Blood Types & Compatibilities

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January is National Blood Donor Month. If you’ve ever donated blood (or received blood from a donation) then you might have wondered why some types of blood can’t be given to some people. And what is it that makes blood types different anyway? This graphic looks at the compatibilities of different blood types, and below we’ll try to get to the bottom of what makes them different.

There are actually lots of different blood types. The International Society of Blood Transfusion recognises 35 blood group systems. We commonly refer to just two of these systems when classifying blood type: the ABO system and the Rh system.

Both systems relate to substances called ‘antigens’ found on the surface of red blood cells. In the ABO system, these antigens are carbohydrate (sugar) molecules. A person’s red blood cells can have only A antigens, in which case they would have blood type A. If they have only B antigens, they would have blood type B; if they have both A and B antigens, they would have blood type AB. It’s also possible to have neither of these antigens, in which case they would have blood type O.

The second blood type system, the Rh system, is a little more straightforward. If a person has Rh protein antigens, they are ‘Rh positive’; if not, they are ‘Rh negative’. For example, a person with an O+ blood type has Rh antigens, whereas a person with an O– blood type does not. There are actually 50 different types of blood antigen included in the Rh system, but when terming blood type as positive or negative we’re referring specifically to one known as the D antigen.

These antigens are very important when considering the compatibility of blood types for transfusions. This is because, in our blood plasma, we also have substances called antibodies. These are proteins that form part of our bodies’ natural defence system. Antibodies match up to specific antigens and can bind to and destroy them. People who don’t have A antigens on their red blood cells have A antibodies in their blood plasma, and people who don’t have B antigens have B antibodies. 

This is a problem for transfusions involving red blood cells. Someone with a B blood type will have A antibodies in their blood plasma. If they receive blood containing red blood cells from an A blood type donor, these antibodies will attack the A antigens. This can cause an adverse reaction due to the immune response, which can be fatal. Rh antibodies are not present in the plasma of Rh negative blood groups but can developed if a person with one of these blood groups receives blood from a Rh positive donor.

As a consequence of this, people with some blood types can only receive blood from people with certain other blood types. The O– blood group can be given to any other blood group, as it doesn’t contain A, B, or Rh antigens. Similarly, as the AB+ blood group has all types of antigens (A, B, and Rh), it doesn’t contain antibodies against them and so can receive blood from any other blood group.

As the graphic below shows, blood types aren’t evenly distributed through the population. Worldwide, the AB– blood group is the rarest, though there are variations in different parts of the world and for different ethnicities. It’s also worth remembering that the ABO and Rh systems only account for some of the possible antigens in blood; there are actually over 600 known antigens. In cases where a person has a particularly rare combination of antigens in their blood, it can be difficult to find a compatible donor.

Blood type distributions

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Blood types are why it’s so important that a range of people donate blood. It’s safe for most people who are fit and healthy to do so, and there is further guidance provided on the websites of the NHS (for donors in the UK) and the American Red Cross (US). Even if you’ve not donated before, National Blood Donor month gives you a good excuse to consider it!

If it’s more information on blood you’re after, we’ve discussed the chemistry of blood previously, including what makes it red, and the chemical that gives it its characteristic metallic odour.





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References & Further Reading