• A
• B
• AB
• O

The surface of red blood cells from Type A blood carries a protein (antigen) called the A-antigen. If this blood were transfused to a person without this antigen, that person's immune system would recognize the Type A blood as foreign and produce an immunulogic response, destroying the blood, and possibly causing an overwhelming (potentially lethal) systemic response in the recipient. Thus, it becomes important to know a potential transfusion recipient's major blood type, so that it can be matched to a donor's blood type.

Type A blood is found in 41% of North Americans. Type B blood (carrying the B-antigen) is found in 9% of North Americans. Type AB blood (carrying both the A and B antigen) is present in 4% of North Americans, and Type O blood (neither the A nor B antigen) is found in 46% of North Americans. Type O has been classically labled as a "universal donor" since it can be given to any recipient (because it carries neither the A nor B antigen). Type AB has been called the "universal recipient" since those individuals can accept blood from any donor. However, blood typing is more complex than this.

The Rh factor (Rhesus factor) may be present (Rh positive) or absent (Rh negative) in any of the major blood groups. (Someone could be "A-positive" or "A-negative".) The Rh factor is another surface protein that can also cause immunulogic reactions if not correctly matched, although the reactions are generally milder and less likely to be fatal.

In addition to the major blood groups and Rh factor, there are other, minor blood groups (more than 50), most of which have little clinical significance.

Whenever blood is tested for its' blood type and Rh, the major blood group and Rh factor will be reported.

Whenever blood is tested for potential transfusion, it may be screened for any unusual antigens/antibodies (type and screen). It may also be cross-matched with the potential recipient (cross-match). In this case, not only are the major blood groups and Rh factor examined, but as many of the important minor blood groups are matched as possible.

Since there are so many minor blood groups, it is unlikely that a blood bank would ever find a "perfect" match. Instead, the major type and Rh is matched, along with a good percentage of the minor blood groups. Transfusion reactions are still possible from minor blood group incompatibility, but the risk is low and the intensity of the reaction usually mild. However, the more blood transfusions that are given to an individual, the more important these minor blood group matches become.

Normal Values*

*These are general values taken from a variety of sources. The actual normal values may vary from lab to lab and from one type of testing protocol to another.