Hi @alxlo1. You’re in excellent hands at Fred Hutchinson. Their doctors, especially Dr Don Thomas, were instrumental in the research and development of bone marrow transplantation. So their reputation is well founded. ☺️

Side effects from the allogeneic transplant can vary from patient to patient. The most serious comes from a reaction of the newly infused cells from a donor called Graft Vs Host Disease or GVHD. This can be skin related issues, organ damage, mouth sores, dry eyes, etc.. Most are mitigated at the beginning.
Bear with me and I’ll explain a little. Then I’ll put a link it with stories from BMT patients including mine.

The bone marrow is the blood manufacturing site in the body, housing blood stem cells which go on to develop into specific blood cells such as red/white and platelets. It is also the home of your immune system, which basically attempts to protect you from any invader such as cancer cells, fungi, bacteria, viruses…. Right now, your current immune system is ineffective and the best bet is to replace it, in hopes that a donor’s immune system will be better.
The preconditioning chemo before the transplant will wipe out your existing marrow and immune system, preparing a clean environment for the upcoming transplanted stem cells.
The new donor stem cells are infused just the same as a blood transfusion, from a little bag attached to an IV line. These little cells, referred to as the graft, are foreign to your body, which is now referred to as the host. They are healthy and ready to do their job. Once infused into the blood, they tumble along the outside walls of the veins until they find specific little pores that open into the bone. Once inside the marrow, these thousands of cells intuitively set up housekeeping (called engraftment) and start manufacturing healthy blood cells again. However, this is NOT your immune system. It’s someone else’s that is now circulating in your body! You can imagine how startled these aggressive little cells are! So they set off all kinds of bells and whistles in panic as they now seek to destroy their new host!! They look at your body as the invader.

But…and it’s a big but, part of the medication regimen you’ll be receiving along with the transplant are anti-rejection drugs which will keep those assertive cells in a very compliant state until they can play well together with your body. It takes several months or longer, but the immune system is adaptive. And it will eventually recognize the foreign proteins and settle down with an amicable partnership. It took me longer, about 2.5 years until I was off the meds. Most people it’s around 3 to 6 months. Like I said we are all different.

I’m going to post a link to a discussion I began several years ago with sharing my BMT story. You’ll find many accountings of fellow members who have gone through the same journey for various blood cancers.
~My bone marrow transplant story: Will you share yours? https://connect.mayoclinic.org/discussion/my-bone-marrow-transplant-bmt-story-will-you-share-yours/

Looking at all the information online can be overwhelming and intimidating. That’s why I posted here. It can be so helpful talking to someone who has gone through this same situation.
Please let me know if there’s anything I can help you with, ok?