Which blood type is considered the universal donor?

The blood type considered the universal donor is O negative (O-). This designation arises from the absence of A and B antigens on the surface of red blood cells, which means that individuals with O- blood can donate to any other blood type without the risk of an immune reaction. When blood is transfused, the recipient’s immune system can recognize the ABO antigens as foreign, leading to complications if the donor blood has incompatible antigens. Since O- lacks these antigens, it can be safely given to patients of any blood type, making it critical in emergency situations when the blood type of the recipient may not be immediately known.

Additionally, O- blood also lacks the Rh factor, further minimizing the risk of transfusion reactions from recipient blood which might not have been sensitized to the Rh antigen. This characteristic is why O- is often referred to as the "universal donor," and it plays a vital role in transfusion medicine.