For each organ in the mature body, there are specific stem cells that can make all the different kinds of cells in that organ. For example, in the blood system, hematopoietic (blood-forming) stem cells (HSC) give rise to each of the different types of blood cells such as red blood cells (RBC), white blood cells (WBC) and platelets. Traditionally, HSCs were obtained from the bone marrow. This process was called “bone marrow transplantation.” However, new methods now obtain HSC from peripheral blood, or blood taken from the placenta at birth (cord blood). Cord blood, in particular, provides an excellent alternative source of HSC for the immune and blood systems. The process of taking HSCs from one person and transfusing them into another is called hematopoietic stem cell transplantation, or HSCT. Unlike transplantation of a solid organ (such as a kidney or liver), HSCT does not involve surgery. It is more similar to a blood transfusion. But instead of just blood, the fluid transfused contains HSCs. The primary immunodeficiency diseases for which HSCT is most commonly performed include Severe Combined Immune Deficiency (SCID), Wiskott-Aldrich Syndrome (WAS), IPEX Syndrome, Hemophagocytic Lymphohistiocytosis (HLH) and X-linked Lymphoproliferative Disease (XLP). It can also be used in the treatment of Chronic Granulomatous Disease (CGD) and many other severe primary immunodeficiency diseases. The transplantation of HSCs from a “normal” individual to an individual with a primary immunodeficiency disease has the potential to replace the deficient immune system of the patient with a normal immune system and, thereby, affect a cure.