There are two primary types of hematopoietic stem cell transplantation: An autologous stem cell transplantation, and an allogeneic stem cell transplantation. The difference between the two types is simple: the terms allogeneic and autologous merely refer to the source of the stem cells: autologous stem cells come from the patient, and they are harvested in advance of the procedure. Allogeneic stem cells are harvested from a matched donor.
Autologous stem cell transplantation
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When oncologists recommend an autologous stem cell transplantation for lymphoma patients, it is typically the last line of therapeutic defense against an otherwise refractory cancer. The idea behind this complicated procedure is rather simple: doctors remove healthy blood stem cells from the patient's body and freeze them so that they can then submit the patient to levels of chemotherapy and radiation high enough to kill the cancerous cells present in the body without affecting their stem cells. Afterwards, those stem cells are returned to the body, where they can develop into the various blood cells required for normal function, including the crucial lymphocytes. In doing so, they rebuild the patient's immune system.
The success of the procedure depends upon the efficacy of the radiation and the chemotherapy regimen in freeing the body of all remaining traces of cancer.
Because the patient is briefly without an active immune system, and because it takes time for the stem cells to rebuild the immune system, the patient is at risk of a wide range of opportunistic infections. During this time, it's not uncommon for old viral infections, such as Herpes or the Chicken Pox, to be re-activated because they are not being kept in check by the immune system. Fortunately, doctors can prepare for such events and mitigate their impact.
Sources
UpToDate.com, HSCT Patient Information
The National Marrow Donor Program
