Success of transplantation between identical twins proposes that the success rate depends on the amount of sharing of histo compatibility genes.

Histocompatibility genes are responsible for the production of antigens on cell surface. With reference to the surface antigens, the grafts or transplants are differentiated in to four types.

They are as follows:

1. Auto graft or Autogenic graft,

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2. Isograft or Syngraft or Syngenetic graft,

3. allografts or Homografts,

4. Xenograft.

1. Auto graft or Autogenic graft:

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When tissue is transplanted from one site to another in the same individual, the transplant is referred as “auto graft” or “autogenic graft” (From Greek Auto=Self).

Immune system of recipient accepts the auto graft very easily, because antigens of recipient cells and the transplanted tissue are alike.

2. Isograft or Syngraft or Syngentic graft:

The graft taken from a genetically identical person is known as Isograft or Syngraft or Syngentic graft. This kind of transplantation is possible between two genetically identical twins.

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Since development of identical twins takes place from a single zygote, identical twins share same genes that are responsible for the production of antigens.

3. Allograft or Homograft:

If the transplantation is carried between genetically different members of the same species then graft is called as “allograft”. The allograft is formally named as “Homograft”.

The histo compartbility antigens of allograft are dissimilar with the host histo compartbility antigens. Hence immune system of recipient/ host identifies the graft as foreign and induces an immune response against to it, resulting rejection of graft.

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4. Xenograft:

If the transplantation between individuals of two different species is carried, for eg. Transplanting monkey liver to human, the graft is referred as “Xenograft”.

Since the histocompatibility genes are quite different, hosts body rejects the graft more vigorously.