What can cause a fever after a blood transfusion?

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Close-up illustration of multiple human red blood cells with their characteristic disc shape, floating against a red background. — blood group The major component of human blood is the red blood cell.

A fever following a blood transfusion, often referred to as a febrile reaction, is primarily caused by the recipient's immune system reacting to donor components. Specifically, patients who receive multiple transfusions may develop antibodies against donor leukocytes, which triggers a brisk febrile response.1

To prevent these reactions, medical professionals often use leukoreduced blood components, which have had the white blood cells removed. While other transfusion-induced immune reactions exist—such as allergic responses, hives, or severe anaphylaxis due to IgA deficiency—these are distinct from the specific mechanism that causes a post-transfusion fever.1