The top panel shows the traditional human fetal Thy/Liv-grafted model with poor in vivo immunity due to inefficient human dendritic-cell (DC) and B-cell development. The bottom panel shows that intravenous injection of CD34⁺ fetal liver cells (FLCs) leads to multilineage repopulation of human lymphohematopoietic cells, formation of secondary lymphoid organs, and development of a functional human immune system in human fetal Thy/Liv-grafted NOD/SCID mice.