is not known exactly why these defects occur. However, it is believed that the defect or error occurs during fetal development. As further research is carried out, different kinds of immune defects are being identified as the cause of the SCID phenotype. In cases of specific enzyme deficiency, immunodeficiency occurs because of a build up of metabolic poisons in the lymphocytes. These children may present at slightly older age than other infants with SCID perhaps because it takes time for the metabolic poisons to accumulate. This form of Severe Combined Immunodeficiency is also inherited through an autosomal recessive pattern, (two abnormal genes, one from each parent, are required).The treatment of choice for these children is bone marrow or stem cell transplantation. The ideal donor is a tissue matched relative. Matching is determined by testing the blood cells for surface proteins HLA-A, HLA-B, and HLA-D, with HLA-D being the most important match to insure significant survival. Prior to transplantation, the patient may undergo irradiation ( form of radiation therapy) or immunosuppressive chemotherapy (a group of chemicals that reduce the chances of rejection) to insure survival of the donated bone marrow or stem cell. This measure is taken to prevent graft-versus-host disease (a reaction of the engrafted tissue against the recipient) and to insure that all new B - and T - cells arise from the donor's normal bone marrow stem cells. In the absence of a tissue matched sibling, patients can be given a T - cell depleted bone marrow transplant from a relative or other partially matched donor. However, graft - versus - host disease (reaction of the engrafted tissue against the recipient) or incomplete reconstitution can occur if no identical normal donor is available. Other sources of donor stem cells can come from the blood of the donor, if procedures are used to boost the number of stem cells that are present. An additional source of st...
