T-lymphocyte deficiency is a feature of which syndrome?

• A. X-linked agammaglobulinemia
• B. Wiskott-Aldrich syndrome
• C. Severe combined immunodeficiency
• D. DiGeorge Syndrome

Answer: (D)

T-lymphocyte deficiency occurs when the thymus is absent or underdeveloped, since T cells mature there. In DiGeorge syndrome, failure of the third and fourth pharyngeal pouches to develop leads to thymic aplasia and markedly reduced T-cell numbers, causing impaired cell-mediated immunity. This manifests as increased susceptibility to viral and fungal infections and problems with T-cell–dependent immune responses. DiGeorge also features hypocalcemia from parathyroid aplasia and commonly congenital heart defects, which helps distinguish it clinically.

Other options involve primarily B-cell defects or broader or different immune problems. X-linked agammaglobulinemia is a B-cell deficiency with very low immunoglobulins and normal T-cell numbers. Wiskott-Aldrich syndrome has T-cell functional defects along with thrombocytopenia and eczema but is not defined by a primary T-cell deficiency. Severe combined immunodeficiency can present with T-cell (and often B-cell) dysfunction, but the classic association with thymic aplasia and specific pharyngeal- pouch origin points to DiGeorge.