Therapeutic complications, such as alloimmunization against RBC in 20-50% of patients or hematopoietic stem cell transplantation (HSCT) graft rejection, constitute an immune-based clinical issue in SCD. Poorly understood RBC alloimmunization is responsible for serious hemolytic transfusion reaction associated with severe mortality and morbidity underlying the need for a better understanding of the immunology of SCD to improve SCD transfusion support/outcome. Little evidence exists about HU effects on immune functions in SCD. HU treatment doesn't appear to have deleterious effects on immune function and appears to decrease the abnormally elevated number of total WBC and lymphocytes, while CT does not.