ABSTRACT
Advances in cellular and molecular immunology have heralded the application of novel
modalities in clinical and experimental transplantation. The successful outcome of bone
marrow and solid organ transplantation has been greatly enhanced in the last 40 years
not only by improvements in surgical techniques, but by progressive understanding of
HLA histocompatibility and typing, by the understanding of transplantation immuno-
biology, and by the development of effective immunosuppressive drugs. Activation of
both T and B lymphocytes is essential for the generation of an antigen-specific immune
response, as is the participation of dendritic and other cells. The inappropriate or undesired
activation of these cells can lead to autoimmunity or, in the setting of allograft transplan-
tation, graft rejection or graft-vs.-host disease (GVHD). T-lymphocyte function is central
to the maintenance and regulation of the immune response and of tolerance; T cells not
only mediate cytotoxicity and immune suppression, but through the production and
secretion of cytokines, T cells also control the proliferation of B cells as well as their
differentiation to antibody-secreting cells. Furthermore, a specific subset of regulatory T
cells appears to be critical for the maintenance of immune homeostasis and tolerance.
Therefore, regulation of the T-cell arm of the immune response is essential for the main-
tenance of immune homeostasis and for the prevention of graft rejection and GVHD.
