The o, A, B and AB parts relate to proteins on the surface of the red blood cells, there are also + and – types of each which relates to the rhesus factor, which is another protein (there also a whole host of other factors but these have less of an effect on transfusion). So, A has the A protein, B the B, AB has A and B and o has none. These proteins act as flags to tell the immune system that your blood is from your body, if the immune system sees the blood as ‘foreign’ it will attack the blood cells. However, you can mix blood types if necessary under certain circumstances, for example AB+ is known as the universal acceptor because it does not have antibodies against any of the proteins and o- is the universal donor because it has no proteins on the cells’ surface for immune cells to react with. So AB+ people can get blood from any person and o- people can donate blood to any person. You could also mix A+ and o+, B+ and o+, I think.