Engraftment is the successful integration of new cells into the patient’s body following a stem cell or bone marrow transplant. This outcome is a significant milestone in recovery, especially after intensive conditioning treatment to eliminate diseased cells and suppress the immune system. Engraftment is necessary for the patient to rebuild a healthy, functioning blood and immune system.
Understanding the Concept of Engraftment
Engraftment is the process where transplanted hematopoietic stem cells travel through the bloodstream to the recipient’s bone marrow, their natural home. Once the cells arrive, they must settle into the bone marrow’s supportive microenvironment, a process known as homing. Successful homing allows the stem cells to begin multiplying and maturing into all types of new blood cells, effectively taking over the role of the patient’s original blood-producing system.
Engraftment is the subsequent event where the cells survive and begin to function, distinct from the initial infusion of stem cells. The new cells produce white blood cells, red blood cells, and platelets, restoring the body’s ability to fight infection and clot blood. Sustained hematopoiesis, or blood cell production, is the ultimate goal of the procedure.
Confirmation and Timeline of Successful Engraftment
Clinicians confirm successful engraftment by monitoring the patient’s peripheral blood counts for specific, measurable thresholds. The most common standard for determining myeloid engraftment is the absolute neutrophil count (ANC). A patient is considered to have engrafted when the ANC reaches a sustained level of 500 cells per microliter (or 0.5 x 10⁹/L) for three consecutive days.
Platelet engraftment is defined as a platelet count of over 20,000 cells per microliter (or 20 x 10⁹/L) for at least seven days without the need for a platelet transfusion. The timeline for reaching these milestones varies, but engraftment usually occurs within 10 to 30 days following the stem cell infusion. Neutrophils are generally the first cells to recover, followed later by red blood cells and platelets.
What Happens When Engraftment is Delayed or Fails
When engraftment does not occur within the expected timeframe, it is termed delayed engraftment, and if it fails entirely, it is known as graft failure. Graft failure is classified as either primary, where engraftment is never achieved, or secondary, where a functioning graft declines or is lost after initial recovery. The lack of new, healthy blood cells leaves the patient in a state of prolonged aplasia, which means they are highly vulnerable to severe infections and bleeding.
This complication significantly increases the risk of transplant-related mortality, necessitating extensive supportive care. Patients require frequent transfusions of red blood cells and platelets, along with powerful antibiotics to manage the constant threat of infection. Factors contributing to failure can include poor stem cell quality, viral infections, or the recipient’s immune system rejecting the donor cells. In cases of sustained graft failure, the patient may require a second stem cell transplant or a donor lymphocyte infusion to attempt to rescue the function of the bone marrow.