Two pathways for generation of T lymphocytes after hematopoietic stem cell transplantation (HSCT).

Shown at top is the adoptive transfer of post-thymic T cells which then undergo expansion in the host, resulting in a limited repertoire of mature T lymphocytes that can rapidly confer antigen-specific responses, including graft-versus-host disease (GVHD). Below is shown the pathway for differentiation of pre-thymic progenitors by migration into the recipient thymus, resulting in the slow generation of a diverse repertoire that is host-tolerant.