This is actually a very deep question, and it strikes right to the heart of evolutionary thinking, although I can’t go into detail here.
The main reason we have different blood groups is because people with different blood groups have a different ability to resist contracting many different parasites, baterial infections and viruses. The diversity of blood groups, and their geographical distribution, reflect the different diseases that different groups of humans have struggled with in the past.
Your blood type depends on which combination of antibodies and antigens are found in your blood. Antibodies are proteins in your plasma (the colourless liquid part of the blood) that are produced by your immune system to seek and destroy harmful organisms such as viruses. Antigens are proteins found on the surface of your red blood cells that cause your immune system to leap into action. There are about 20 different recognised blood group systems and your blood group is inherited so it stays the same for your entire life.
Scientists don’t yet fully understand why humans (and animals) have evolved different blood groups. Some blood groups are associated with particular diseases – for example in the 13th century it appears that people with blood type A were more likely to die from the plague. Different blood groups are more common in different parts of the world – group O is the most common group in the UK.
People in the O blood group are known as universal donors. This is because their blood doesn’t contain any antigens so won’t be rejected by the immune system of the recipient. However, if you tried to give A blood to someone from the B blood group it could kill them as their body would start making antibodies to try and destroy the ‘foreign’ A antigens. In fact, we only know about the existence of different blood groups because doctors noticed patients often died when given blood transfusions.
Do you know what blood type you are? If you sign up to be a blood donor (you have to be 17) they will tell you what blood group you are in. Look here for more information: http://www.blood.co.uk/
I ought to know that. I think we were told when I studied immunology, but I can’t remember. I think it’s probably because different blood groups evolved to suit different environments. I know that there is one blood type that helps to protect against malaria.