Which blood type is considered the universal donor?

Type O blood is considered the universal donor because it lacks A and B antigens on the surface of its red blood cells. This absence of antigens means that Type O blood can be safely transfused to individuals with any other blood type without triggering an immune response.

When blood type A, B, or AB is transfused, the recipient's immune system can react against the foreign antigens present in those blood types, potentially causing serious complications. Since Type O does not present these antigens, it can be accepted by people with any blood type (A, B, AB, or O), making it an essential and highly valuable resource in emergency situations where blood compatibility may be unknown.