Allogeneic blood transfusion is a form of temporary transplantation. This procedure introduces a multitude of foreign antigens and living cells into the recipient that persist for a variable time. A recipient who is immunocompetent may mount an immune response to the donor antigens (ie, alloimmunization), resulting in various clinical consequences, depending on the blood cells and specific antigens involved. The antigens most commonly involved can be classified into the following categories: (1) human leukocyte antigens (HLAs), class I shared by platelets and leukocytes and class II present on some leukocytes; (2) granulocyte-specific antigens; (3) platelet-specific antigens (human platelet antigens [HPAs]); and (4) RBC-specific antigens.
The consequences of alloimmunization to blood-based antigens include the following clinical manifestations:
Alloimmunization against RBCs
Acute intravascular hemolytic transfusion reactions (rarely a consequence of alloimmunization and almost always caused by ABO antibodies)
Delayed hemolytic transfusion reactions (DHTRs) (hemolysis caused by RBC alloantibodies typically presenting clinically 7–14 days after transfusion)
Hemolytic disease of the fetus and newborn (mother’s alloimmunization against red blood cell fetal antigens, most often resulting from previous pregnancies)
Alloimmunization against platelets (platelet-specific or HLA class I antigens)
Refractoriness to platelet transfusion (an increase in the platelet count after platelet transfusion that is significantly lower than expected [eg, < 30% of predicted 10-60 min posttransfusion or < 20% at 18-24 h posttransfusion])
Posttransfusion purpura (thrombocytopenia after transfusion of red cells or other platelet-containing products, associated with the presence of platelet alloantibodies)
Neonatal alloimmune thrombocytopenia (mother’s alloimmunization against fetal platelet antigens, most often resulting from previous pregnancies but can be seen in a first pregnancy)
Alloimmunization against granulocytes (granulocyte-specific or HLA antigens)
Refractoriness to granulocyte transfusion
Febrile nonhemolytic transfusion reactions
Transfusion-related acute lung injury (ie, a transfusion reaction in which donor HLA antibodies react with recipient white blood cell antigens)
Alloimmunization against HLA antigens
Alloimmunization against blood cell antigens in bone marrow transplantation leading to hemolysis and the possibility of delayed engraftment
Hemolytic transfusion reactions, posttransfusion purpura, febrile nonhemolytic transfusion reactions, and transfusion-related acute lung injury are discussed in Transfusion Reactions. Hemolytic disease in newborns and neonatal alloimmune thrombocytopenia are discussed in other sections of Medscape Reference. Transplant rejection is discussed in Assessment and Management of the Renal Transplant Patient.
DHTRs and refractoriness to platelet transfusions are discussed in this article. Refractoriness to granulocyte transfusion involves either HLA or granulocyte-specific antibodies and is similar to platelet refractoriness, except that refractoriness to granulocyte transfusion results in the patient failing to respond clinically to the infused granulocytes. Because granulocyte transfusions are rarely used, they are not discussed further in this article.