D. B cell defectDiGeorge Syndrome {Thymic Hypoplasia) Robbins 9th/24lDiGeorge syndrome is a T-cell deficiency that results from failure of development of the third and fourth pharyngeal pouches. The latter give rise to the thymus, the parathyroids, some of the C cells of the thyroid, and the ultimobranchial body. Thus, individuals with this syndrome have a variable loss of T cell-mediated immunity (resulting from hypoplasia or lack of the thymus), tetany (resulting from lack of the parathyroids), and congenital defects of the heart and great vessels.Absence of cell-mediated immunity is caused by low numbers of T lymphocytes in the blood and lymphoid tissues and poor defense against certain fungal and viral infections.The T-cell zones of lymphoid organs--paracortical areas of the lymph nodes and the periarteriolar sheaths of the spleen--are depleted. Ig levels may be normal or reduced, depending on the severity of the T-cell deficiency."Patients who survives the neonatal period show enhanced susceptibility to viral, fungal and bacterial infections, which ultimately prove fatal"- Ananthanarayan 10th/175Patients with DGS may have poor T-cell production compared to their peers, and as a result, have an increased susceptibility to viral, fungal and bacterial infections- primaryimmune.orgDiGeorge Syndrome Harrison I9th/2483A form of hypoparathyroidism associated with defective development of both the thymus and the parathyroid glands is termed the DiGeorge syndrome, or the velocardiofacial syndrome.Congenital cardiovascular, facial, and other developmental defects are present, and patients may die in early childhood with severe infections, hypocalcemia and seizures, or cardiovascular complications.Most cases are sporadic, but an autosomal dominant form involving microdeletions of chromosome 22q11.2 has been described.