Ontogeny of cellular immunity in man

The purpose of this study was to determine the precise developmental sequence of the acquisition of phytohemagglutinin (PHA) and allogenic cell responsiveness (MLR) in the human fetus. Reactivity was determined quantitatively by 3H-thymidine incorporation in various fetal lymphoid tissues. MLR reactivity was the earliest detectable response occurring first in liver lymphoid cells at 5 weeks fetal age. Reactivity to PHA then appeared in thymocytes at 10.5 weeks and at 14 weeks in the spleen and blood. MLR reactivity first appeared in the thymus at 12.5 weeks and in the spleen and blood at 14.5 weeks. Marrow lymphocytes failed to react up to 18 weeks. These results suggest that fetal thymus is immunocompetent by about 10.5 weeks and MLR responsiveness may be under nonthymic (hepatic) control during development.