What is the blood type known as the universal donor?

The blood type known as the universal donor is O negative (O-). This designation comes from the absence of A and B antigens on the surface of red blood cells in type O blood, which means that when O- blood is transfused into patients with any other blood type, there is a lower risk of the recipient's immune system reacting against the donated blood.

In blood transfusions, compatibility between the donor and recipient blood types is crucial because the presence of A or B antigens can trigger an immune response if the recipient’s immune system identifies them as foreign. Since O- blood lacks these antigens, it can be safely given to individuals with A, B, AB, or O blood types without the risk of an incompatible reaction.

Moreover, O- blood is also Rh negative, which means it does not have the Rh factor, another antigen that can complicate transfusions. Consequently, O- is often used for emergency transfusions when there is no time to determine a patient's blood type.

In summary, the unique compatibility of O- blood, owing to the absence of both A/B antigens and the Rh factor, makes it the universal donor in blood transfusions.