How does O negative compatibility work with the Rh factor?

O negative (O-) blood is known as the "universal red blood cell donor" because it is compatible with almost any blood type. This compatibility is based on two factors: the ABO system and the Rh system.

1. The "O" (ABO System): Blood types A, B, and AB have "antigens" (specific protein markers) on the surface of their red blood cells. Type A has A antigens, Type B has B antigens, and Type AB has both. Type O blood is "O" because it has no A or B antigens. Because it lacks these markers, a recipient's immune system (whether they are Type A, B, or AB) has no antigens to attack, so it won't reject the blood.
2. The "Negative" (Rh Factor): The Rh factor is another antigen. If you have this antigen, your blood is "Rh positive" (+). If you lack it, you are "Rh negative" (-). O negative blood lacks the Rh antigen.

How it works together: An O negative red blood cell has no A antigens, no B antigens, and no Rh antigens. Because it is "empty" of these three main antigens, it can be safely given to a patient with any blood type (A+, A-, B+, B-, AB+, AB-, O+, or O-) without triggering a major immune system attack. This makes it invaluable in emergencies when there is no time to determine the patient's blood type.