I'm going to explain the process that Derek is going to go through and then I'll tell you my thought.
Derek's bone marrow is fibrous. Scar tissue has built up inside of his bones making it so his red blood cells can't occupy that space. That's the reason they have traveled to his spleen. (that's a side issue though) His cells aren't dividing correctly. He's basically not producing enough red blood cells, platelets, and white blood cells.
So the process to cure it is to give his chemotherapy to kill off all of his bad cells. Once they've done this they will then take stem cells that they have collected from Brian (Derek's brother) who is a match and will transfuse them into Derek. The cells will then go where they need to go and start to divide and produce. They will also attack any lingering bad cells and fill up the bone marrow space. The amazing thing is that Derek now has Brian's immune system. Brian's cells are Derek's now. The inside of Derek's body isn't his anymore. He essentially gets a new identity inside.
I was thinking about this and God had me realize something. That's what all of this is! The trials we go through, the storms we weather are spiritual chemotherapy. God is killing us inside (in a good way)! He is getting rid of our baser natures. He is creating space so He can fill us. The bad cells and the good cells can't occupy the same space. When that happens, we slowly begin to die. Every time I pray and every time I read my Word another bad cell is killed thus leaving more space for Him. We need to be cleaned out, in order for Christ to infuse us with His blood. Blood that was given freely so that we may experience eternal life! God is good!