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Summary Studies of human thymus cells in short-term (3 to 91 hr) tissue cultures have shown the stimulation produced by phytohemagglutinin (PHA). This has been demonstrated by increased uptake of isotopically labeled DNA, RNA and protein precursors measured by autoradiography and liquid scintillation counting. The appearance of “blast cells” after culture with PHA has been shown by electron microscopy. There was wide variability from the one thymus cell preparation to another with regard to the dose of PHA and the concentration of cells per culture required for maximal isotope uptake. Thymus cells from older patients appeared to respond better to PHA than did thymus cells from younger patients. A comparison of cultures of peripheral blood leukocytes with cultures of human thymus cells indicates that at least some thymus cell preparations respond to PHA as well as do peripheral blood leukocyte preparations.

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