Lymphocytes in blood samples taken from the thymic, brachiocephalic and internal jugular vein of children undergoing thoracic surgery were compared regarding their phenotypic markers and their proliferative response to mercuric chloride (HgCl2) and phytohemagglutinin (PHA). Of these two mitogens, PHA stimulated the DNA synthesis of the lymphocytes from the thymic vein to a significantly higher degree than those taken from the brachiocephalic or internal jugular vein. The response to HgCl2 resulted in the highest stimulation of the DNA synthesis in cells from the thymic and the brachiocephalic veins, though the differences were not significant. The higher reactivity to PHA and the tendency to higher reactivity to HgCl2 of lymphocytes taken from efferent thymic blood might be due to a higher frequency of T cells. This, however, could not be verified by immunohistochemical staining, as Leu4-positive cells occurred in the same proportion in all three venous samples. We therefore suggest that the passage through the thymus of circulating T cells may increase their immune reactivity by the influence of thymic maturation hormones.
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