Blood transfusion reactions

Blood transfusion reactions are adverse responses to transfused blood products, most of them immune-mediated. They are classified by timing (acute versus delayed) and mechanism, and the major types map neatly onto the hypersensitivity framework tested in immunology.

The most feared is the acute hemolytic transfusion reaction, a type II hypersensitivity usually caused by ABO incompatibility — typically from a clerical error. Preformed recipient antibodies destroy donor red cells within minutes to hours, producing fever, flank pain, hypotension, hemoglobinuria, and potentially disseminated intravascular coagulation and renal failure. The febrile nonhemolytic reaction is far more common and benign: recipient antibodies against donor leukocytes, or accumulated cytokines in the stored unit, cause fever and chills within a few hours. Leukoreduction of blood products reduces its incidence.

Allergic (urticarial) reactions are type I responses to plasma proteins, causing hives and pruritus; severe anaphylactic reactions classically occur in IgA-deficient recipients with anti-IgA antibodies, requiring washed blood products. Transfusion-related acute lung injury (TRALI) — donor anti-leukocyte antibodies activating neutrophils in the recipient's pulmonary capillaries — presents as acute respiratory distress and noncardiogenic pulmonary edema within six hours and is a leading cause of transfusion-related death. Delayed hemolytic reactions arise days to weeks later from an anamnestic antibody response to minor red cell antigens such as Kidd or Rh.

USMLE Step 1 tests transfusion reactions within immunology: match each reaction to its mechanism, timing, and classic clinical vignette — ABO mismatch for acute hemolysis, IgA deficiency for anaphylaxis, and respiratory distress for TRALI.