CD56+CD3- NK Cells Following Allogeneic Stem Cell Transplantation

Inclusion Criteria: 1. Patients undergoing an allogeneic HSCT from a sibling donor, as treatment for a hematological malignancy. The conditioning regimen, and in particular whether ablative or non ablative, will not be considered in the criteria for recruitment 2. Patient and donor Age \>18 years 3. Patients and donors must have signed an informed consent form 4. The donor must be willing and capable of donating lymphocytes for NK selection using apheresis techniques 5. Donor must be fit to undergo leukapheresis Exclusion Criteria: 1. Life expectancy \< 3 months 2. ECOG performance status 3 or 4 3. Uncontrolled intercurrent illness including, but not limited to ongoing or active infection, symptomatic congestive heart failure, unstable angina pectoris, life threatening cardiac arrhythmia 4. Patients will not be eligible if they receive in vivo T depletion with ATG, ALG or campath-1H 5. HIV-positive patients 6. Psychiatric illness/social situations that would limit compliance with study requirements and ability to comprehend the investigational nature of the study and provide informed consent