Robertson D. Davenport, associate professor of pathology and medical director of the Blood Bank and Transfusion Service at the University of Michigan, explains.

There are several important differences between blood transfusions and organ transplants. Transfused red cells are expected to last for no more than three months, while transplants will hopefully function for many years. Transfusions are given intravascularly, and transplants are, of course, implanted. Immune responses to foreign antigens that are given intravascularly tend to be less pronounced than responses to antigens introduced through other routes. Transplanted organs contain some immune cells from the donor that can stimulate the recipient, whereas most immune cells that might be in a transfusion are filtered out before administration.

Blood transfusions may be rejected by the recipient, resulting in a transfusion reaction, but such cases are relatively rare. In order to comprehend how this can happen, it is necessary to understand some basic immunology. There are two basic types of immune responses: humoral, or antibody-mediated, and cellular. Humoral immune responses result in the production of antibodies that are specific to a foreign antigen. When these antibodies attach to the antigen--on bacteria, for example--immune complexes are formed. The body's macrophages, mainly in the liver and spleen, then remove and destroy the immune complexes. Once attached to antigens, antibodies can also activate what is known as the complement pathway. As an end result, complement activation can punch holes in the membranes of bacteria or cells that are coated with antibodies. When red blood cells are the target of antibodies and complement, a condition called hemolysis results. Immune responses evoked by blood transfusion, when they occur, are usually humoral in nature. Organ and bone marrow transplants, in contrast, usually evoke cellular immune responses, which lead to the production of specific cytotoxic lymphocytes.

The most important blood cell antigens in transfusion are in the ABO system. Blood types can be A (having A but not B antigens on red cells), B (having B but not A antigens), AB (having both A and B antigens), or O (having neither). Virtually everyone over the age of six months has antibodies to the A or B antigens that they don't produce. For example, a group A individual has A antigen on his red cells and anti-B in his plasma. If blood is transfused to a patient who already has antibodies to the transfused cells (giving this individual group B blood, for example) then a serious reaction can occur. Because antibodies to A and B antigens are good at activating complement, transfusion of ABO incompatible red cells can cause breakdown of the cells in circulation and a strong inflammatory response. The end result can be kidney failure and even death. Fortunately, this type of immediate rejection of transfused red cells is rare.

A more common type of rejection of transfused red cells is a delayed hemolytic reaction. In this case, the patient does not have preexisting antibodies to the transfused red cells. Rather, an immune response occurs days to weeks after the transfusion. The antibodies produced in such reactions tend not to activate complement, so the transfused red cells in circulation do not usually break down. Instead, the cells are removed by the spleen and a milder inflammatory response may occur. Delayed hemolytic reactions occur in about one out of every 5,000 transfusions. The antibodies involved can be directed against one or more of hundreds of known blood group antigens. Delayed hemolytic transfusion reactions usually are not very severe, but sometimes do cause renal failure. Patients with sickle cell anemia are at greater risk of having delayed hemolytic reactions because they often receive many transfusions. These reactions can be more severe than in other patients because the loss of normal hemoglobin containing red cells and the inflammation caused by the reaction can trigger a sickle crisis.

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