Final answer:
An allograft is the transplant of an organ or tissue between genetically different individuals of the same species, often requiring immunosuppressive drugs to prevent rejection.
Step-by-step explanation:
An allograft refers to the transplantation of tissue or an organ from one individual to another within the same species but with different genetic makeup. In contrast, a xenograft is when tissues are transplanted between different species. Allografts encounter the challenge of immune rejection because the recipient's body recognizes the transplanted organ as foreign due to differences in major histocompatibility complex (MHC) proteins, prompting an immune response. Matching the MHC proteins between donor and recipient as closely as possible, along with the use of immunosuppressive drugs, can help reduce the risk of rejection and extend the lifespan of the graft.