During the initial chemo session, the white cells are taken to nadir (0) and hopefully the blasts (bad white cells) are totally eliminated. When they come back, which usually takes a couple of weeks the good cells are taken and saved for future use. If the chemo doesn't work and the white cells don't recover, then you use the good cells that were taken. The need for someone elses cells comes when the first chemo treatment fails and the white cells never come back.
Got it. Thanks for the explanation.