At week 28 (4 weeks after the fourth injection), patients undergo apheresis, a procedure for collecting large numbers of white blood cells. The cells are tested in the laboratory to see how the immune system responds to the study vaccine. For the procedures, blood is collected through a needle in an arm vein and spun in a machine that separates the white blood cells from the rest of the blood (red cells, plasma and platelets). The white cells are removed and the rest of the blood is returned to the patient's body through the same needle. Patients who cannot or do not want to undergo apheresis have 80 mL (about 1/3 cup) of blood drawn through a needle and collected in blood collection tubes.