A bone marrow transplant (BMT), more accurately called a hematopoietic stem cell transplant, is a medical procedure used to treat blood cancers or genetic disorders. It involves replacing a patient’s unhealthy, blood-forming tissue with healthy stem cells that generate all types of blood cells. This procedure can change a person’s blood type, but only under specific circumstances related to the donor’s and recipient’s blood groups. This change occurs because the new stem cells take over the production of all future blood components.
The Role of Stem Cells in Blood Identity
The potential for a blood type shift stems from the fundamental source of all blood cells within the body. Red blood cells, which carry the markers that determine a person’s blood type (such as the A, B, and Rh antigens), are constantly being produced and replaced. This continual production process is managed by specialized cells in the bone marrow called hematopoietic stem cells (HSCs).
These stem cells are the ultimate progenitors for every cell found in the blood, including red cells, white cells, and platelets. The ABO and Rh markers that define a person’s blood type are genetically determined by the HSCs. When a patient undergoes a bone marrow transplant, the goal is to replace the patient’s native HSCs with healthy donor HSCs. If the donor’s HSCs successfully engraft and begin producing new blood cells, every new red blood cell generated will carry the donor’s blood type markers, effectively changing the recipient’s blood identity over time.
The Conditions Required for a Blood Type Shift
A change in blood type only occurs when the donor and the recipient have different ABO or Rh blood groups, known as a “mismatched” transplant. If both individuals share the same blood type, the new cells will simply match the old ones, and no change will be observed. The ABO system (types A, B, AB, and O) and the Rh system (positive or negative status) are the most relevant factors in this shift.
For instance, if a recipient with type A-positive blood receives a transplant from a donor with type O-negative blood, the patient’s new stem cells will produce O-negative red blood cells. Once the transition is complete, the patient will be considered O-negative for all medical purposes involving blood products. This process can create a temporary state called “chimerism,” where the patient has a mixture of their original and the donor’s new blood type cells circulating simultaneously.
Mismatched transplants are categorized as major, minor, or bidirectional, depending on the presence of antibodies that could react against the other’s blood components. In a major mismatch, the recipient has antibodies that could attack the donor’s red blood cells. A minor mismatch involves the donor’s plasma having antibodies against the recipient’s red blood cells. Despite these potential complications, modern transplant techniques allow for successful transplants across these blood type barriers, and ABO incompatibility occurs in a significant percentage of all transplants.
Monitoring the New Blood Type
The conversion to the donor’s blood type is not an instantaneous event but a gradual, carefully monitored process. After the donor’s stem cells are infused, they must travel to the bone marrow and begin to multiply, a process called engraftment. Engraftment typically begins within a few weeks of the transplant, but the full shift in blood type takes much longer.
The complete replacement of the recipient’s original red blood cells by the donor’s cells generally occurs over several weeks to many months, often ranging from three to twelve months. During this period, clinicians use specialized blood tests, including blood cell counts and chimerism testing, to track the progress of the new cells. Chimerism testing determines the proportion of donor cells to recipient cells in the blood and bone marrow.
Regular blood typing is performed to confirm the permanent shift to the donor’s ABO and Rh type, ensuring the new cells are fully established and functional. Until this conversion is complete and confirmed, the patient may show a mixed blood type profile, which requires careful management of any necessary blood transfusions.
Implications for Future Transfusions
Once the blood type conversion is complete and the donor’s stem cells are fully responsible for blood cell production, the patient’s transfusion needs permanently change. The patient must receive blood transfusions that match their new blood type (the donor’s type), rather than their original type. For example, a patient who was originally A-positive but converts to O-negative must receive O-negative blood products going forward.
Blood bank professionals must be fully aware of the patient’s transplant history and current blood status to prevent life-threatening transfusion reactions. This requirement applies to all subsequent red blood cell transfusions for the rest of the patient’s life. While the blood type changes due to the replacement of blood-forming cells, other aspects of the recipient’s biology, such as the tissue type of their organs, remain unchanged.