Based on the presence or absence of a special antigen A or B, there are four types of blood, viz. A, B, AB and O. Blood group A has antigen A, blood group B has antigen B, blood group AB has both antigens and blood group O has none. RBCs also contain another antigen which is called Rh factor. Blood with Rh factor is called Rh positive, while one without Rh factor is called Rh negative. About 80% people in the world have Rh positive blood.