The intravenous injection of absorbed (erythroagglutinin free) PHA in mice resulted in a marked increase in spleen weights, numbers of nucleated spleen cells, and spleen cells in mitosis. The morphologic changes induced in the splenic lymphocytes were striking and closely paralleled the changes observed following the addition of PHA to cultures of human and animal leukocytes. Observations on the absolute numbers of mature lymphocytes and immature lymphoid and blast cells suggest that the proliferative effect of PHA on spleen lymphocytes is due in part to mitosis following blast cell transformation of mature small lymphocytes. PHA also stimulated the normoblasts and granulocytes present in the mouse spleen to proliferate. The normoblasts showed a single early peak of proliferative activity. The granulocytes showed a similar early increase in numbers, but remained elevated for the duration of the experiment. The lymphoid cells showed an early, primary peak of proliferation as well as a later, secondary, smaller peak which preceded the appearance of serum antibody directed against PHA. The findings suggest that PHA acts as both a nonspecific or nonantigenic stimulus and as an antigenic stimulus to the proliferation of spleen lymphoid cells in vivo.

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