Do you mean blood type....like can be O positive or O negative?
If that is what you mean, that is the Rh factor. It is a protein that you either have in your blood or do not, due to genetics. If you are + you have it, - you do not. It is the reason why someone with negative blood can not receive positive blood, while someone with positive blood can receive negative. A person with - there body would recognize the protein as foreign and attack it. The same goes for a mother with negative blood, carrying a child with positive blood......they are given a shot while pregnant and another at the birth of the child to suppress that immune responce.
If that is what you mean, that is the Rh factor. It is a protein that you either have in your blood or do not, due to genetics. If you are + you have it, - you do not. It is the reason why someone with negative blood can not receive positive blood, while someone with positive blood can receive negative. A person with - there body would recognize the protein as foreign and attack it. The same goes for a mother with negative blood, carrying a child with positive blood......they are given a shot while pregnant and another at the birth of the child to suppress that immune responce.