Explain why, in principle, an organ transplanted from any donor other than an identical twin is almost certain to be rejected in the absence of any other treatment

Question

What will be an ideal response?

Answer 1

Acute rejection is due chiefly to immune responses made by the recipient's T cells against HLA class I and II molecules of the graft that are different from those of the recipient and that the recipient's immune system perceives as ‘foreign.' The differences can be due to the HLA molecules, the self peptides they bind, or both. Transplantation between identical twins and transplantation of autografts are the only situations in which the graft and the recipient are genetically identical and in which there are no differences in either the HLA molecules or the bound peptides. In these situations, graft rejection does not occur. Transplantation between donors and recipients who have identical HLA class I and II molecules, usually HLA-identical siblings, almost always involves differences in the peptides that are bound by the HLA molecules. These differences trigger peptide-specific alloreactive T cells to cause graft rejection through the direct pathway of allorecognition. Although it is possible to match donor and recipient for many HLA class I and II allotypes, in practice most clinical transplants involve one or more mismatched HLA loci. For these differences in HLA type, alloreactive T-cell clones activated by either the direct or indirect pathway of allorecognition cause graft rejection. Destruction of the grafted organ is effected through a type IV delayed-type hypersensitivity response.