 On Day -5, you will receive fludarabine by vein over 1 hour and cyclophosphamide by vein over 3 hours. If you have a B-cell cancer, you will receive rituximab by vein over 3-6 hours. Rituximab and fludarabine are given to treat the cancer and lower the immune system in order to lower the risk of the body rejecting the NK cells. Cyclophosphamide is given to lower the immune system in order to lower the risk of graft-versus-host disease GVHD -- when transplanted immune tissue, such as cord blood NK cells, attacks the tissues of the recipients body. 