Transfusion Reactions

A transfusion reaction is an adverse event occurring during or after the transfusion of blood components. Reactions range from common and mild, such as febrile non-hemolytic and allergic reactions, to rare and life-threatening, such as acute hemolytic reactions from ABO-incompatible transfusion, transfusion-related acute lung injury (TRALI), and transfusion-associated circulatory overload (TACO). Many of the most dangerous reactions are immune-mediated, which links them directly to immunohematology.

Definition

Transfusion reactions are adverse responses to the transfusion of blood or blood components, encompassing immune-mediated reactions (such as hemolytic, febrile non-hemolytic, allergic, and TRALI) and non-immune hazards (such as circulatory overload), classified by their timing and underlying mechanism.

Scope

This entry classifies transfusion reactions by mechanism and timing — acute versus delayed, immune versus non-immune — and explains the principal immune reactions including acute and delayed hemolytic reactions, febrile non-hemolytic reactions, allergic and anaphylactic reactions, and TRALI, alongside non-immune hazards such as TACO. It is a reference overview of why and how these reactions occur, not a management protocol.

Core questions

• How are transfusion reactions classified by mechanism and timing?
• What causes an acute hemolytic transfusion reaction and why is ABO error the classic trigger?
• How do TRALI and TACO differ as causes of post-transfusion respiratory distress?
• Why are immune reactions central to transfusion safety?

Key concepts

• Acute vs delayed reactions
• Immune vs non-immune mechanisms
• Acute hemolytic transfusion reaction (ABO incompatibility)
• Delayed hemolytic transfusion reaction (anamnestic antibody)
• Febrile non-hemolytic transfusion reaction
• Allergic and anaphylactic reactions
• Transfusion-related acute lung injury (TRALI)
• Transfusion-associated circulatory overload (TACO)
• Hemovigilance

Mechanisms

Immune transfusion reactions arise when recipient or donor antibodies react with antigens on transfused or recipient cells. In an acute hemolytic reaction, pre-existing recipient antibodies — classically anti-A or anti-B following an ABO mismatch — bind donor red cells, activate complement, and cause intravascular hemolysis. Delayed hemolytic reactions reflect an anamnestic rise in alloantibody against a previously sensitized red-cell antigen, producing extravascular hemolysis days later. Febrile non-hemolytic reactions are attributed to recipient antibodies against donor leukocytes or to accumulated cytokines, and allergic reactions to recipient antibodies against plasma proteins. TRALI is mediated by donor anti-leukocyte antibodies or biologically active lipids that prime and activate recipient neutrophils in the lung. The antiglobulin (Coombs) test and antibody identification are used to characterize the immune reactions, tying transfusion safety to blood-group serology.

Clinical relevance

Recognizing transfusion reactions is fundamental to transfusion safety, and immune reactions in particular connect bedside events to the underlying immunohematology. This entry describes the categories and mechanisms of reactions so that students and clinicians can understand them; it does not provide instructions for managing a reacting patient or for administering blood.

Epidemiology

Mild reactions such as febrile non-hemolytic and allergic reactions are the most frequently reported, while acute hemolytic reactions are rare but carry high mortality and are most often caused by clerical or identification errors leading to ABO-incompatible transfusion. TRALI and TACO are leading causes of transfusion-related death in hemovigilance reporting, which has driven mitigation strategies in blood-component preparation and clinical practice.

History

Severe reactions to early blood transfusions were recognized once ABO incompatibility was understood after Landsteiner's discovery of the blood groups, and the introduction of compatibility testing greatly reduced fatal hemolytic reactions. Over the twentieth and twenty-first centuries, attention broadened from hemolysis to non-hemolytic immune hazards such as TRALI and to circulatory overload, which modern hemovigilance systems now track as leading causes of transfusion-related harm.

Key figures

• Karl Landsteiner
• Robin Coombs
• Meghan Delaney
• Jeanne Hendrickson

Seminal works

• delaney-2016
• hendrickson-2009

Frequently asked questions

What is the most dangerous type of transfusion reaction?

Acute hemolytic transfusion reactions from ABO-incompatible red cells are among the most dangerous, causing rapid intravascular hemolysis; they are rare and usually result from identification errors, while TRALI and circulatory overload are leading causes of transfusion-related death.

Are all transfusion reactions immune-mediated?

No. Many serious reactions such as hemolytic, febrile non-hemolytic, allergic, and TRALI reactions are immune-mediated, but others such as transfusion-associated circulatory overload are non-immune.