Ontogeny of cellular immunity in the human fetus: Development of responses to phytohemagglutinin and to allogeneic cells

Human fetal lymphoid cells from thymus, spleen, blood, liver, and bone marrow of 22 fetuses (5–19 weeks of fetal age) were studied for their ability to respond to phytohemagglutinin and to adult allogeneic lymphocytes by the mixed lymphocyte reaction. The earliest detectable response was that by hepatic cells in the mixed lymphocyte reaction at 7.5 weeks of fetal age. Phytohemagglutinin reactivity was initially seen in the thymus at 10 weeks, in blood at 14.5 weeks, and in spleen at 13 weeks. Mixed lymphocyte reaction reactivity was first detected in the thymus at 12.5 weeks; it appeared somewhat later in peripheral organs. Few significant responses to phytohemagglutinin were detected in bone marrow or hepatic cells, and virtually no response to allogeneic cells was found in bone marrow. All lymphoid cells studied showed stimulatory ability in the mixed lymphocyte reaction. However, spleen, blood, and marrow cells produced higher stimulation of allogeneic cells than did thymic or hepatic cells.