Yes, your body can reject a blood transfusion. Your immune system may recognize donated blood cells as foreign and attack them, destroying them in a process called a hemolytic transfusion reaction. This is uncommon thanks to modern blood typing and screening, but it does happen, and it can range from a mild fever to a life-threatening emergency.
How Your Body Attacks Donated Blood
A transfusion reaction starts when your immune system produces antibodies against proteins on the surface of donated red blood cells. Everyone’s red blood cells carry specific surface markers, and if the donated blood has markers your body doesn’t recognize, your immune system treats those cells the same way it would treat a virus or bacteria. The antibodies latch onto the foreign cells and trigger their destruction, releasing their contents into your bloodstream. That flood of cellular debris can damage your kidneys, disrupt blood clotting, and cause a dangerous drop in blood pressure.
The most well-known markers are the ABO blood group (the reason you’re typed as A, B, AB, or O) and the Rh factor (positive or negative). Mismatches in these major groups cause the most severe reactions. But there are also dozens of minor surface markers, including ones in the Kell, Kidd, Duffy, and MNS blood group systems. These minor markers are what make transfusion reactions possible even when the major blood type is a perfect match.
Acute Reactions: Within Hours
Acute hemolytic reactions happen within 24 hours of receiving blood, sometimes within minutes. Early warning signs include fever, flushing, a sudden drop in blood pressure, anxiety, wheezing, and dark or red-colored urine. The red urine comes from hemoglobin spilling into the bloodstream as red blood cells are destroyed. In severe cases, the reaction can trigger widespread, uncontrolled clotting throughout the body, which is a medical emergency.
These acute reactions are rare. In large hospital surveillance studies, acute hemolytic reactions occur at a rate of roughly 0.03 per 1,000 transfusions. The most common transfusion reactions by far are mild allergic responses (hives, itching) and febrile reactions (fever and chills without actual blood cell destruction), which together account for the vast majority of adverse events.
Delayed Reactions: Days to Weeks Later
Not all rejection happens right away. Delayed hemolytic reactions typically show up 5 to 15 days after a transfusion, though they can appear as late as 30 days afterward. These occur when your body has a “memory” immune response to a minor blood group marker it encountered during a previous transfusion or pregnancy. At the time of the original exposure, your body formed antibodies, but those antibodies faded to undetectable levels. When you receive blood with that same marker again, your immune system ramps back up and destroys the transfused cells.
Symptoms of a delayed reaction include jaundice (yellowing of the skin and eyes), dark urine, pallor, and sometimes pain and fever that can mimic other conditions. Because these symptoms develop gradually and days after leaving the hospital, delayed reactions can be tricky to recognize.
Lung Complications From Transfusions
Two distinct lung problems can develop after a transfusion. Transfusion-related acute lung injury (TRALI) is an immune-mediated reaction where the lungs suddenly fill with fluid, making it hard to breathe. It typically involves a drop in blood pressure and is not caused by fluid overload. Antibodies from the donor’s blood are thought to activate white blood cells in the recipient’s lungs, damaging the blood vessel walls there.
Transfusion-associated circulatory overload (TACO) looks similar but has a different cause: too much fluid entering the bloodstream too quickly, essentially overwhelming the heart. TACO tends to cause high blood pressure and responds well to medications that help the body eliminate excess fluid, while TRALI causes low blood pressure and does not improve with the same approach. Both require immediate medical attention and stopping the transfusion.
When Donor Cells Attack Your Body
One of the rarest and most dangerous transfusion complications is transfusion-associated graft-versus-host disease. In this scenario, living white blood cells from the donated blood survive in your body, recognize your tissues as foreign, and mount an immune attack against you. It’s the reverse of a typical rejection: instead of your body attacking the donated blood, the donated blood attacks your body.
This condition typically develops 2 to 30 days after transfusion and causes fever, skin rash, jaundice, diarrhea, and dangerously low blood cell counts as the donor’s immune cells destroy the recipient’s bone marrow. It carries a mortality rate above 90%. People with weakened immune systems and those who receive blood from donors who happen to share certain genetic markers are at the highest risk. To prevent it, blood products for high-risk patients are irradiated, which kills the donor’s white blood cells while leaving the red blood cells functional.
Who Faces the Highest Risk
The more transfusions you receive over your lifetime, the more likely your immune system is to develop antibodies against minor blood group markers. This process, called alloimmunization, is a cumulative risk. People with sickle cell disease are among the most affected populations, with alloimmunization rates between 20% and 50%. Part of the reason is a mismatch in blood group marker frequencies between patients, who are predominantly of African descent, and the general blood donor pool, which in many countries is predominantly white. When donor and recipient populations share more similar ethnic backgrounds, as is often the case with thalassemia patients, alloimmunization rates drop to around 10%.
Women are also at higher risk than men, partly because pregnancy itself can trigger antibody formation. If the baby carries blood markers inherited from the father that the mother doesn’t have, her immune system may produce antibodies during delivery. Those antibodies then lie in wait for a future transfusion carrying the same markers. People who have had organ transplants or who receive ongoing transfusion therapy for chronic conditions accumulate risk with each exposure.
How Hospitals Prevent Reactions
Before any transfusion, your blood is typed for ABO group and Rh factor. Then a crossmatch test is performed: a sample of your plasma is mixed with a sample of the donor’s red blood cells in the lab to see if your antibodies react. If the cells clump together or are destroyed, the blood is incompatible and won’t be used. A negative crossmatch result, meaning no reaction, is required before transfusion can proceed.
For patients who have already developed antibodies against minor blood group markers, the lab runs an expanded antibody screen to identify exactly which markers to avoid. This is especially important for people with sickle cell disease or others who need repeated transfusions. Some transfusion programs now provide extended antigen matching for these patients, selecting donor blood that matches not just the ABO and Rh groups but also the Kell, Duffy, Kidd, and other clinically significant markers.
What Happens If a Reaction Starts
The first and most important step is stopping the transfusion immediately. The IV line is kept open with saline to maintain access and support blood pressure. Medical staff then assess airway, breathing, and circulation. For mild reactions like fever or hives, treatment may be as simple as a fever reducer or antihistamine. For severe reactions involving breathing difficulty, a dangerous blood pressure drop, or signs of red blood cell destruction, the response escalates quickly: fluids are pushed to protect kidney function, oxygen is started, and the identity of the patient and the blood product are double-checked to rule out a labeling or clerical error.
The blood bag and a fresh blood sample from the patient are both sent to the lab for testing. This helps determine whether the reaction was immune-mediated and identifies the specific antibody involved, which is critical information for any future transfusions you might need. Most transfusion reactions, when caught early, are manageable. The key is vigilance during and immediately after the transfusion, which is why nurses typically monitor you most closely during the first 15 minutes, when acute reactions are most likely to appear.