s (one of the five kinds of white blood cells, or leukocytes, circulating in the blood) arise, are absent, or defective. Little or no antibody production occurs. If the T - helper cells are absent or function poorly, normal functioning of T - and B - lymphocytes are impaired. If the thymus gland is absent or functioning poorly, this impacts negatively on the normal maturation process for T-helper and T-suppressor cells. In the absence of fully developed T - lymphocytes, the immune system can never function normally. If stem cells (or precursor cells for T- and B-cells) in the bone marrow are defective or absent, this eliminates or damages the original source of T - and B - lymphocytes. It 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).Little or no antibody production occurs. If the T-helper cells are absent or function poorly, normal functioning of T - and B - lymphocytes are impaired. If the thymus gland is absent or functioning poorly, this impacts negatively on the normal maturation process for T - helper and T - suppressor cells. In the absence of fully developed T - lymphocytes, the immune system can never function normally. If stem cells (or precursor cells for T- and B-cells) in the bone marrow are defective or absent, this eliminates or damages the original source of T - and B - lymphocytes. It...
