Why Do We Have Two Different Blood Types?

Imagine your red blood cells are like little balls with sticky patches on them. Some people have sugar patches and others do not. If you get a transfusion from someone with different patches, your body might mistake their blood for invaders and attack it.

Your body makes special guards called antibodies. These guards patrol your bloodstream looking for the wrong kind of patches. Type A blood has 'A' sugar on top. Type B has 'B' sugar. Type O has neither. If you have Type A blood, your guards are happy because they see their own sugar. But if you get Type B blood, the guards shout 'Intruder!' and start a fight.

You might wonder why we didn't just pick one perfect type for everyone. Evolution is messy. Different types of bacteria and viruses attack our bodies in different ways. Having variety means that when a new sickness comes along, some people will survive because their blood type resists that specific germ.

• A baby born with a mismatched blood group might get yellow skin until doctors help it out.
• You can give your own blood back to yourself after surgery without rejection.
• People in different parts of the world often have favorite types because local diseases shaped them over thousands of years.