Immunodeficiency in Down's syndrome: low levels of serum thymic factor in trisomic children.

The activity of thymus-dependent serum factor SF was significantly lower in 18 children wit Down's syndrome (DS) than in 14 matched controls. The percentage of circulating T lymphocytes forming E-rosettes was also low in DS. Together with the previous finding of immature of T lymphocytes in peripheral blood, the present data suggest that the basic immune defect of DS is failure in differentiation of peripheral post-thymic precursors to fully immunocompetent T lymphocytes resulting from lack of thymic hormonal factors.