Why does someone have blood type O?
Each blood group has a different antigen. Antigen is a special protein produced by the body’s immune system to identify potentially dangerous foreign cells.
The antigen then combines with the antibodies in the blood plasma to form unique molecular combinations. This combination of antigens and antibodies determines your blood type. Type O blood does not have A or B antigens, but it does have A and B antibodies
In addition, blood group also has an additional antigen called rhesus (Rh factor). If your blood is found to have the Rh factor, it means that your blood is “positive”, and it is written with a “plus” sign, such as A +, B +, AB +, and O +.
On the other hand, if your blood does not contain rhesus, it means your blood type is “negative” so it is marked with a minus symbol (-), such as A-, B-, AB- or O-.
The antigen is inherited from your father and mother. You can get type O blood if one or both of your parents have O blood.