A transfusion reaction is any unwanted response your body has to receiving donated blood or blood products. These reactions range from a mild fever to rare, life-threatening emergencies. In the United States, about 282 reactions occur for every 100,000 units of blood transfused, and the vast majority are manageable. Understanding what these reactions look like and why they happen can help you know what to expect if you or someone close to you needs a transfusion.
Why Transfusion Reactions Happen
Your immune system is built to recognize what belongs in your body and attack what doesn’t. When donor blood enters your bloodstream, your body sometimes identifies proteins, white blood cells, or other components in that blood as foreign. This triggers an immune response that can cause symptoms ranging from chills to organ damage, depending on the type and severity of the mismatch.
Not all reactions are immune-driven, though. Some are non-immunologic, meaning they happen for mechanical or volume-related reasons. Receiving blood too quickly, for instance, can overload your circulatory system. Bacteria that contaminated a blood product during storage can cause infection. The distinction matters because immune reactions and non-immune reactions look different, progress differently, and require different responses.
Febrile Reactions: The Most Common Type
Febrile non-hemolytic reactions account for roughly 120 out of every 100,000 transfusions, making them by far the most frequent. The hallmark is a fever of 38°C (100.4°F) or higher, or a temperature rise of more than 1°C from your baseline, during or within four hours after the transfusion. You might also experience chills, shaking, headache, or nausea.
These reactions happen through two main pathways. In one, your body produces antibodies against proteins on donor white blood cells. In the other, immune-signaling molecules that accumulated in the blood product during storage provoke the response. Neither pathway destroys red blood cells, which is why these reactions are called “non-hemolytic.” They’re uncomfortable but not dangerous, and they typically resolve on their own or with simple fever-reducing treatment.
A blood processing step called leukoreduction, which filters out most white blood cells from donated blood before storage, has significantly reduced the rate of febrile reactions. There is moderate-quality evidence supporting this practice. Interestingly, the common hospital habit of giving fever reducers and antihistamines before a transfusion has not been shown to reliably prevent these reactions, and some experts question whether routine premedication is necessary at all when leukoreduced blood is used.
Allergic Reactions
Mild to moderate allergic reactions are the second most common type, occurring about 88 times per 100,000 transfusions. These typically involve hives, itching, or skin flushing and are caused by your immune system reacting to proteins in the donor’s plasma. They usually respond well to antihistamines and don’t require stopping the transfusion permanently.
Severe allergic reactions, including full anaphylaxis, are far rarer, at roughly 2.5 per 100,000 transfusions. One well-known risk factor is IgA deficiency, a condition affecting between 1 in 200 and 1 in 500 people. IgA is an immune protein found in plasma. People who lack it can develop antibodies against IgA, and when they’re exposed to donor blood containing IgA, those antibodies can trigger anaphylaxis. Anyone who has had a severe allergic reaction during a transfusion will typically have their IgA levels tested afterward. Those confirmed to have anti-IgA antibodies need specially selected blood products for any future transfusions.
Acute Hemolytic Reactions
This is the reaction most people picture when they think of a “bad transfusion,” and it’s also one of the rarest. Acute hemolytic reactions caused by ABO blood type incompatibility (receiving the wrong blood type) occur about 0.2 times per 100,000 transfusions. When they do happen, they can be life-threatening and require immediate action.
The classic triad of symptoms is fever, flank pain, and red or brown urine. The urine changes color because antibodies in your blood attack the donor red blood cells, rupturing them and releasing their contents into your bloodstream. Your kidneys filter out the released hemoglobin, which tints the urine. You might also feel a burning sensation at the IV site, chest tightness, nausea, agitation, or sudden shortness of breath. Heart rate often spikes while blood pressure drops. If the reaction progresses, it can lead to kidney failure or widespread abnormal blood clotting.
ABO incompatibility is the most common cause, but reactions can also involve other blood group systems like Kell or Duffy. The moment a hemolytic reaction is suspected, the transfusion is stopped immediately. Hospital staff will verify the patient’s identity against the blood product label, because the most common root cause is a clerical or identification error somewhere in the process.
Lung-Related Reactions: TRALI and TACO
Two types of transfusion reactions primarily affect the lungs, and while they can look similar at first, they have very different causes.
TRALI (transfusion-related acute lung injury) is an immune reaction where antibodies in the donor blood damage the tiny blood vessels in your lungs, causing fluid to leak into lung tissue. The result is sudden difficulty breathing, low oxygen levels, and fluid visible on a chest X-ray. Critically, this happens without your heart being overloaded. TRALI occurs about 1.5 times per 100,000 transfusions, but it carries serious consequences. Between 2018 and 2022, TRALI and possible TRALI accounted for 18% of all transfusion-related deaths reported to the FDA.
TACO (transfusion-associated circulatory overload) is a volume problem, not an immune problem. It happens when blood is transfused faster or in greater volume than the heart and circulation can handle. Fluid backs up into the lungs, causing breathing difficulty that looks a lot like TRALI on initial assessment. TACO occurs more frequently, about 11.7 per 100,000 transfusions, and has become the leading cause of transfusion-related deaths in the U.S., responsible for 34% of reported fatalities from 2018 through 2022. Older adults and people with existing heart conditions are at highest risk. Doctors distinguish between the two conditions using heart function tests and a blood marker that tends to be elevated in TACO but not in TRALI.
Delayed Reactions
Not all transfusion reactions happen while the blood is flowing. Delayed hemolytic reactions typically show up 5 to 21 days after the transfusion. They occur when your immune system “remembers” a foreign blood cell protein it encountered in a previous transfusion or pregnancy, mounts a fresh antibody response, and begins destroying the transfused red blood cells over the following days and weeks.
The symptoms are often subtle. You might notice unexplained fatigue, mild jaundice (yellowing of the skin or eyes), or a drop in the blood counts that the transfusion was meant to improve. Many delayed reactions are caught only through routine lab work that shows signs of red blood cell breakdown. Delayed serologic reactions, where new antibodies form without obvious red cell destruction, are more common, occurring about 18.6 times per 100,000 transfusions. Actual delayed hemolytic reactions with symptoms happen around 4.8 times per 100,000.
Transfusion-Transmitted Infections
Modern blood screening has made infections from transfusions exceedingly rare. Viral transmission occurs at a rate of roughly 0.04 per 100,000 units transfused. Bacterial contamination is slightly more common, at about 0.23 per 100,000 units, with platelet products carrying the highest risk because they’re stored at room temperature, which allows bacteria to grow. Sepsis from contaminated platelets is reported in up to 1 in 2,300 platelet transfusions, though not all of these cases become severe.
What Happens During a Suspected Reaction
If you develop symptoms during a transfusion, the nursing staff will stop the infusion right away. This is standard protocol for any suspected reaction, no matter how mild the symptoms seem. Your vital signs will be checked, and the blood product bag along with your identification will be rechecked to rule out a labeling or patient-matching error. Blood and urine samples are typically collected to look for signs of red blood cell destruction.
For mild reactions like fever or hives, treatment is straightforward and the transfusion can often be restarted once symptoms are controlled. For serious reactions involving breathing difficulty, a significant blood pressure drop, or signs of hemolysis, the transfusion is not restarted. Treatment shifts to stabilizing your breathing, blood pressure, and organ function. The blood bank will run additional testing on both your blood and the donor unit to identify what went wrong and guide any future transfusion decisions.
How Common Are Life-Threatening Reactions
Overall, life-threatening transfusion reactions requiring major medical intervention dropped from 9.4 per 100,000 units in 2015 to 4.7 per 100,000 in 2017. In fiscal year 2022, the FDA received 54 reports of potentially transfusion-associated deaths across the entire country, and only 30 of those (56%) were classified as definitively, probably, or possibly related to the transfusion itself. Blood transfusion remains one of the most tightly regulated and monitored medical procedures, and the trend in serious complications has been declining for years.