Blood Types

A blood type is a classification of blood based on the presence or absence of inherited antigenic substances on the surface of red blood cells. Blood types are inherited and represent contributions from both parents. A total of 30 human blood group systems are now recognized by the International Society of Blood Transfusion.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.
There's also another antigen - 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. 84% of the population is Rh positive.