We all have these little marker things called antigens on the surface of our red blood cells.
These are so tiny they can't even be seen under a microscope.
Everyone's got different ones - blame your parents and grandparents for all that.
In fact, only identical twins will ever possess all the same antigens.
All that's very well and good, but when it comes to blood transfusions, there are two very important systems of antigens which need to be matched to avoid any nasty complications
The ABO System
If you have blood group A then you've got A antigens covering your red cells.
Blood group B means you have B antigens, while group O has neither, and group AB has some of both.
The ABO system also contains lots of little antibodies in the plasma, antibodies being the body's natural defence against foreign antigens.
So blood group A has anti-B in their plasma, blood group B has anti-A (you probably get the picture at this stage).
To complicate matters though, group AB has none and group O has both of the antibodies.
Which means giving someone blood from the wrong ABO group could be fatal.
The anti-A antibodies in group B attack group A cells and vice versa.
Which is why group A blood must never be given to a group B person.
Group O negative is a different story.
Well, it gets more complicated here on in, because there's another antigen to be considered - the Rh antigen.
Some of us have it, some of us don't.
If it is present, the blood is RhD positive, if not it's RhD negative.
So, for example, some people in group A will have it, and will therefore be classed as A+ (or A positive).
While the ones that don't, are A- (or, wait for it...A negative).
And so it goes for groups B, AB and O.
This effectively doubles the number of different blood types to be matched, because you shouldn't mix blood type A+ with blood type A-.
84% of the population is Rh positive.
(And yes, that means the other 16% of the population is running around with Rh negative blood.)