Most blood transfusions go smoothly, but side effects range from mild fever and itching to rare, serious complications like lung injury or immune reactions. The most common reactions happen during or shortly after the transfusion and are easily treated. Severe reactions are uncommon, and the risk of infection from modern blood supplies is extraordinarily low.
Common Mild Reactions
The side effects most people experience are minor and manageable. These include mild fever, chills or shivering, headache, and hives or itching. A mild fever happens because your immune system responds to the foreign blood cells, even when the blood type is a perfect match. This is called a febrile reaction, and it typically resolves on its own or with a standard fever reducer.
Itching and hives signal a mild allergic reaction to proteins in the donated blood. Nursing staff monitor you closely during the first 15 minutes of a transfusion, which is when reactions are most likely to start. If hives appear, the transfusion may be paused while you receive an antihistamine, then resumed once symptoms settle. These mild reactions rarely require stopping the transfusion entirely.
Acute Hemolytic Reactions
An acute hemolytic reaction is one of the most serious transfusion complications. It happens when your immune system recognizes the donated red blood cells as incompatible and destroys them. Your body produces antibodies that attack the donor cells, breaking them apart in the bloodstream. This is most often caused by a mismatch in blood type, though modern cross-matching procedures have made it very rare.
Signs of a hemolytic reaction include back or flank pain, bloody or dark urine, sudden fever and chills, skin flushing, and dizziness or fainting. These symptoms can begin within minutes of the transfusion starting. If you feel any sudden pain, pressure, or discomfort during a transfusion, tell the medical staff immediately. The transfusion will be stopped right away, and the blood will be tested to confirm what happened.
Lung-Related Complications
Two distinct conditions can affect your lungs during or after a transfusion, and they require different treatment even though they look similar at first.
The first, transfusion-related acute lung injury (TRALI), causes sudden difficulty breathing and low oxygen levels, typically within six hours of the transfusion. Chest imaging shows fluid in both lungs, but the heart is functioning normally. The risk is estimated at roughly 1 in 12,000 transfused units. TRALI is the second leading cause of transfusion-related deaths in the United States, accounting for about 24% of fatal transfusion complications.
The second, transfusion-associated circulatory overload (TACO), happens when the volume of fluid overwhelms your heart’s ability to keep up. It looks similar to TRALI on a chest X-ray, but blood tests show signs of cardiac strain. TACO is actually more common than TRALI and is the leading cause of serious transfusion-related problems, responsible for roughly 34% of transfusion-related deaths. People with existing heart or kidney conditions, older adults, and small-framed individuals are at higher risk because their circulatory systems are more sensitive to extra fluid volume.
Delayed Immune Reactions
Not all immune reactions happen right away. A delayed hemolytic reaction develops 1 to 4 weeks after a transfusion, when your body slowly builds antibodies against proteins on the donated red blood cells. Many people with delayed reactions have no symptoms at all. When symptoms do appear, they tend to be mild: a slight fever, mild jaundice (yellowish skin), or darker urine.
The most telling clinical clue is often an unexplained drop in hemoglobin back to pre-transfusion levels about 1 to 2 weeks after the procedure. In other words, the benefit of the transfusion essentially disappears because the donated cells were destroyed. Severe symptoms like significant pain, chills, and pronounced jaundice are rare. If you notice any of these signs in the weeks after a transfusion, your medical team can run a simple blood test to check for antibodies.
Infection Risk
Modern blood screening has made transfusion-transmitted infections extremely rare. Every donated unit goes through rigorous testing, and the current estimated risks per donation are remarkably low:
- HIV: approximately 1 in 21.4 million donations
- Hepatitis C: approximately 1 in 12.6 million donations
- Hepatitis B: approximately 1 in 7.5 million donations
To put that in perspective, you are far more likely to be struck by lightning than to contract HIV from a blood transfusion. Bacterial contamination of platelet products is a slightly more common concern than viral transmission, because platelets are stored at room temperature. Blood banks use bacterial screening to minimize this risk as well.
Iron Overload From Repeated Transfusions
A single transfusion won’t cause iron overload, but people who receive transfusions regularly, such as those with sickle cell disease or thalassemia, face a cumulative buildup of iron. Your body has no natural mechanism for removing excess iron, so it deposits in organs over time.
The liver is often affected first. Early scarring of the liver has been observed in children with thalassemia as young as age 7, and once cirrhosis develops, the risk of liver cancer increases. The heart is also vulnerable. In patients who receive ongoing transfusions without iron-removal therapy, heart failure develops after an average of about 10 years. More than one third of people with transfusion-dependent thalassemia also develop restricted lung function.
Iron overload disrupts the endocrine system too. It can impair thyroid function, delay puberty in more than half of affected young patients, and interfere with other hormone-producing glands. Even the immune system suffers: excess iron impairs the function of certain white blood cells, making some patients more susceptible to specific infections. Iron-removal therapy (chelation) can slow or reverse some of this damage when started early enough.
Graft-Versus-Host Disease
One of the rarest but most dangerous transfusion complications occurs when immune cells in the donated blood attack the recipient’s tissues. This is called transfusion-associated graft-versus-host disease, and it is fatal in more than 90% of cases, with death typically caused by overwhelming infection.
The people most at risk are those with severely weakened immune systems, particularly defects in the type of immune cells that would normally recognize and destroy the foreign donor cells. Patients undergoing certain cancer treatments, infants with immune deficiencies, and people with Hodgkin’s disease are among those at elevated risk. In genetically homogeneous populations with limited diversity in immune-system markers, the risk rises significantly because the recipient’s body may fail to recognize the donor cells as foreign. The condition can also occur in people with fully healthy immune systems, though this is uncommon. Blood products can be treated with irradiation to destroy the donor immune cells and prevent this complication in high-risk patients.
What Monitoring Looks Like
During a transfusion, your vital signs are checked at baseline, again about 15 minutes after the transfusion starts, and once more after it finishes. That 15-minute check is especially important because most acute reactions surface in that early window. Staff also rely heavily on patient observation: changes in your breathing, skin color, comfort level, and alertness can signal a reaction before vital signs shift.
If you’ve had a transfusion reaction in the past, let your care team know beforehand. They may pre-treat you with fever reducers or antihistamines, use specially processed blood products, or slow the infusion rate. Most reactions are caught early and managed effectively when staff and patients are both paying attention.