Circulating thymic-hormone activity was assayed by measuring Thy 1·2 antigen induction on null lymphocytes from athymic mice incubated with human plasma or serum. Plasma from 19 normal children aged under 10 had inductive activity equivalent to 10·6-16·2 ng thymopoietin/ml. Plasma from 15 infants with severe combined immunodeficiency, 2 of whom had appreciable immunoglobulin synthesis, and from 2 infants with DiGeorge syndrome had little or no inductive activity. Successful reconstitution with thymus or bone-marrow grafts and with redcell infusions (if adenosine-deaminase deficiency is present) was followed by a rise in circulating thymic-hormone activity.
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