Final answer:
Tissue transplantation is facilitated through the matching of HLA molecules between a donor and recipient to reduce the chances of transplant rejection and improve the likelihood of tolerance. Close HLA matching is essential to minimize risks such as graft-versus-host disease and to make immunosuppressive therapy more effective.
Step-by-step explanation:
Tissue transplantation has been made possible by our ability to match HLA molecules between individuals. Human leukocyte antigens (HLAs), which are equivalent to the major histocompatibility complex (MHC) molecules, play a crucial role in transplant rejection. A donor organ expressing MHC molecules different from the recipient often results in a cytotoxic T cell response, leading to rejection. A biopsy showing massive infiltration of T lymphocytes is a sign of likely transplant rejection. To reduce the risk of this immune response, it is essential to match the HLA molecules of the donor and recipient as closely as possible. In some cases, such as bone marrow transplants, the risk of graft-versus-host disease (GVHD) further necessitates close HLA matching. Moreover, transplant recipients typically require lifelong immunosuppressive therapy to improve the chances of tolerance, even with good HLA matching, making them susceptible to infections and transplant-related malignancies.