The appearance of an antibody post-transfusion is a sign of a delayed transfusion reaction. Only when there is evidence of increased red cell destruction is the reaction termed a delayed hemolytic transfusion reaction (DHTR). The appearance of an antibody without cell destruction is a delayed serologic reaction.
Delayed reactions occur more than 24 hours post-cessation of transfusion. Usually, the blood appears serologically compatible at initial testing. Delayed reactions are common in patients who have been immunized to a foreign antigen from a previous transfusion or pregnancy, but the antibody titers decrease over time, and the antibody is not detectable during pre-transfusion testing.
The transfusion leads to a secondary (anamnestic) response, causing increased antibody production that sensitizes antigen-positive donor red cells. Hemolysis is extravascular with a DHTR. Sensitized cells are removed from circulation by the reticuloendothelial system, also called the monocyte-macrophage system. Because there is a delay in the presentation of symptoms, DHTR is not usually considered a cause of the clinical presentation. The transfusion service usually initiates an investigation of a DHTR because of serologic findings in a post-transfusion specimen.
DHTRs occur more frequently than acute hemolytic reactions.
