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Emergency transfusion: Compatibility

The safe transfusion of blood and blood products is dependent upon antigen compatibility. Antigens on red blood cells are carbohydrate-based molecules that are recognized by antibodies. O, A, and B denote the antigens present on red blood cells. If an antigen is not present on the cell, then the antibody to that antigen is present in plasma and available to target that antigen if it becomes present. Therefore, patients that do not express a B antigen will develop an immediate antibody-mediated reaction upon exposure to B-expressing cells. The O antigen forms the scaffolding upon which the A and B antigens are assembled, so type AB patients do not produce antigens to the O antigen. There is a rare blood type that does not express an O antigen; these patients would appear to be type O but would test positive when their plasma is tested against type O red blood cells.

Blood types also include Rh factor. If not present, there will only be antibodies to Rh factor in the plasma if there is a history of exposure to Rh+ blood. If a patient is Rh-, they should only be given Rh- blood to avoid developing anti-Rh antibodies. If a patient is Rh+, it doesn’t matter if the transfused blood is Rh+ or Rh-.

For transfusion of plasma, the rules are inverse to that of red blood cells. For example type A plasma will have anti-B antibodies. Therefore, any blood type can receive type AB plasma and only type O can receive type O plasma.

In an emergency, when crossmatching cannot be performed in time, “emergency release” blood can be used. This is type O- blood. This is used until antibody screening and cross-matching can be performed.

Platelets do not express Rh antigens. However, prepared units of platelets contain small numbers of RBCs. Therefore, it is preferable to use Rh- platelets if the recipient is a woman of childbearing age. They do express ABO antigens and thus should be matched with donor blood in accordance with patient’s blood type.


  1. McCullough J. Overview of platelet transfusion. Semin Hematol 2010; 47:235 PubMed Link