Final answer:
An allograft is the transplantation of tissue from one individual to another within the same species, which requires a close HLA match to minimize the risk of rejection and GVHD. Bone marrow transplants are an example where HLA matching is crucial, and recipients often require immunosuppressive therapy long-term.
Step-by-step explanation:
Tissue that originates from a donor of the same species and requires human leukocyte antigens (HLAs) to be fairly similar for a reasonable chance of success is known as an allograft. In procedures such as bone marrow transplants, the HLAs of the host and donor must closely match to minimize the risk of transplant rejection and graft-versus-host disease (GVHD). These HLAs are part of the major histocompatibility complex, with the three highly polymorphic MHC I genes (HLA-A, HLA-B, and HLA-C) critical in determining compatibility. Due to the high number of alleles in the human population, a perfect match is unlikely without a genetic relation, making siblings or parents often the best donors. The bone marrow is processed before transplantation to remove donor's antigen-presenting cells (APCs) and T cells, leaving primarily hematopoietic stem cells. Most transplant recipients require ongoing immunosuppressive therapy to prevent rejection, even with a good genetic match.