When a doctor describes your blood type, he’ll probably not only indicate its type, 0, A, B, or AB, but also if it’s “positive” or “negative.” This refers to your Rh factor. If your blood is positive, you have the Rh factor; if it’s negative, you don’t.
A person with the Rh factor has certain substances, called antigens, in his red blood cells. Rh negative persons don’t have them. These antigens don’t do any harm, but it’s important for you to know if you have them.
As when a person with Rh negative blood receives a transfusion of Rh positive blood, the two kinds of blood don’t mix well, and the red blood cells break apart. A person with Rh negative blood should receive only blood that is Rh negative.