There are over 20 blood type groups, the most prominent of which is the ABO group. Blood type is determined by the incidence of antigens on the cell membrane. In the ABO grouping system, the RBC cell membrane can have antigen A, B, neither, or both. People with blood type A would have an A antigen. People with blood type B would have a B antigen. People with AB blood have both antigens, and people with blood type O have neither antigen.
The immune system develops antibodies for the antigens it lacks. People with blood type A have an antibody against blood type B, and vice versa. People with blood type AB have no antibodies, so they can accept any type of blood. People with blood type O have antibodies for types A and B.
Why is Type O blood the universal donor? Blood type O is the universal donor because it has no antigens on its RBC membranes. This means that people with blood types A, B, and AB have no antibodies to reject type O. Blood type AB is the universal recipient because it contains no antibodies, and therefore will not reject any of the other blood types regardless of its antigens.
Blood type O is only transfused to a patient if there is no time to match their blood type. It is more ideal to find them blood of the same group.
The notion of the universal recipient is a theoretical one. A person with blood type AB wouldn’t just be given any blood. Blood type O can be given if AB blood is unavailable, or there is no time to retrieve it.
It’s always safer and more practical to transfuse blood of the same type. The ABO blood type is a factor, but also an individual’s Rhesus blood group is typed before any transfusion takes place. Other blood categories are not checked unless the recipient has preexisting issues.
Reference:
https://en.wikipedia.org/wiki/Blood_type