I'll explain it in Ian speak and intersperse few links to far more thorough explanations about terms and the transplant as a whole.
Katie is slated to receive a mini-allogeneic transplant. Basically it's a variation on what's commonly called a bone marrow transplant. Essentially they knock the patient's immune system out with chemo therapy and then take the stem cells (which are concentrated in the bone marrow) of a donor and introduce them into the patient. The goal is to have the donor stem cells replace the immune system of the host.
Before minis, full allogeneic transplants involved an extremely toxic level of chemo. What they recently discovered is that if you give a much lower dose of chemo, the donor immune system can finish off the remaining Leukemia with far less damage to the host.
For more details on how a mini-allo transplant works, check out this explanation.
The donor is anonymous but the details we are allowed to know are all encouraging. They are in their 20s and share 11 of the 12 currently known HLA markers with Katie which all points at a great match. For those wondering, we did test Katie's brother, Peter, to see if he was a match, but unfortunately no dice.
Why now? Unfortunately the last round of chemo (campath) never pushed the CLL into remission and at best it will give Katie 18 months between treatments. With every successive round of treatment, her body takes on more damage and the disease responds less to the chemo. The choice is to go ahead with the bone marrow transplant now while Katie is still strong and healthy in order to give her the best shot at getting through it successfully.
-ian