Hematologic, Immunologic, and Neoplastic Disorders Practice Test 2026 - Free Questions and Study Guide

When ABO incompatibility occurs, the recipient’s preformed IgM antibodies target the donor’s red blood cells and activate complement, causing rapid intravascular hemolysis of the transfused cells. This leads to an acute transfusion reaction that usually appears within minutes to a few hours of starting the transfusion. The classic presentation includes fever and signs of hemolysis such as hypotension from shock, flank or back pain from the kidney’s response to free hemoglobin, and hemoglobinemia with hemoglobinuria (dark urine). Lab clues include low haptoglobin and a positive direct antiglobulin test, reflecting immune-mediated destruction of red cells.

In contrast, a delayed hemolytic reaction typically occurs days after transfusion and is due to different antibodies causing extravascular hemolysis, with milder symptoms like fever and renewed anemia but without the abrupt intravascular destruction. Febrile nonhemolytic reactions feature fever and chills but no evidence of hemolysis. Allergic reactions occur within seconds to minutes and present with urticaria or bronchospasm, without intravascular hemolysis.