Which blood type is considered the universal donor type?

Type O is known as the universal donor blood type because it lacks A and B antigens on the surface of its red blood cells. This absence means that it can be transfused into individuals with any other blood type without the risk of an immune reaction. When a person receives blood, their immune system can react against foreign antigens; since Type O blood does not introduce any new antigens, it is safe for all recipients.

In contrast, other blood types—Type A, Type B, and Type AB—have specific antigens that could trigger an immune response in individuals who do not have the same type. For instance, Type A blood has A antigens that could be attacked by the antibodies present in Type B or Type O recipients. Similarly, Type B blood has B antigens that could cause reactions in Type A or Type O recipients. Type AB, while it can receive from all types, contains both A and B antigens, making it unsuitable as a donor to anyone other than other AB recipients. Therefore, Type O's unique characteristics make it the most versatile option for blood donation across different blood types.