Stem Cell Harvest Stem cells are special cells that can make all the different types of blood cells. Usually they are found in the bone marrow, but after chemotherapy many more of them are in your blood stream. Between the third and fifth cycles of induction, stem cells will be collected from the blood using a procedure called apheresis. The central line or catheter is connected to a machine that draws some of the subjects blood out of one side of the catheter, filters out the stem cells, and returns the rest of the blood through the other side of the catheter. Each apheresis procedure takes about 4-6 hours. The procedure may need to be done several times to collect enough stem cells.