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Blood Type Matching On their surface, red cells have inherited chemical structures called antigens that can cause a person’s immune system to make antibodies against them. Humans have 35 major groups or families of these antigens, as well as other minor groups, but consideration of two, the ABO group and the RhD group, is very important to ensure that a transfusion recipient receives compatible blood. The presence of antigens within these groups is what determines a person’s blood type. Blood types are referred to as Type A, Type B, Type AB (which has both A and B antigens), or Type O (which has neither A or B antigens) followed by positive or negative, which indicates the presence of the RhD antigen. Persons who are RhD negative have no RhD antigen. Blood Types ABO Antigen Group RhD Antigen Group A Positive A Negative B Positive B Negative AB Positive AB Negative O Positive O Negative Persons with a given antigen do not make antibodies against their own antigen, but will against the other antigens in that group. For example, if a person has type A blood, he (or she) will make antibodies against the B antigen, but not against the A antigen. That means that if this person is given either type B red blood cells or type AB red blood cells, his or her antibodies will attack and destroy the foreign red cells possessing the B antigen, causing a transfusion reaction. Similarly, a person having the RhD antigen will not make antibodies against RhD antigen, but the RhD negative person will make antibodies against blood with RhD positive red cells. Recipient Blood Type Matching Donor Blood Type A+ A+, A-, O+, O- A- A-, O- B+ B+, B-, O+, O- B- B-, O- AB+ Compatible with all blood types AB- AB-, A-, B-, O- O+ O+, O- O- O- Antibodies in the plasma of donors with different blood types cause a reverse situation. For example, since a person with Type AB blood makes no antibodies against Type A or Type B cells, his or her plasma can be given to a person with Type A, Type B, Type AB, or Type O blood. In emergency situations, when the recipient’s blood type is unknown, the person can receive type O negative red cells without producing an ABO or RhD incompatibility reaction. In other situations, even non-emergent, where the recipient’s specific blood type is known but there is no type-specific blood available, a compatible type may be used if determined to be compatible by cross-matching. Cross-matching is a simple and commonly practiced laboratory test that verifies blood compatibility. In the latter instance, for example, if an AB positive recipient needs blood and there are no AB positive donors, they could receive blood from a Type A positive donor after a proper cross match. Allergic and Other Reactions Delayed hemolytic reactions may occur if the recipient makes antibodies against minor antigens on the transfused red cells. Such reactions are usually mild and are caused by certain chemicals in the donor’s blood; these may cause fever, hives, rashes, itching, low blood pressure, and similar symptoms. In rare instances, patients receiving massive blood transfusions may develop transfusion-associated acute lung injury, which is caused by an inflammatory reaction to large amount of foreign chemicals in the donor’s blood.