Classification of grafts

Classification of grafts : The graft can be classified into four major types.

1. Autograft : The tissue of the original donor is grafted back into the same donor. For example, skin graft from thigh to face in severely deformed case of burnt individuals (plastic surgery).

2. Isograft : Graft between syngeneic individuals (ie., identical genetic constitutuion). For example, clones or identical twins.

3. Allograft : (Homograft). Graft between allogenic individuals (ie., members of the same species but of different genetic constitution. For example, kidney transplanted from one human to another.

4. Xenograft : (Heterograft). Graft between xenogenic individuals (ie., different genetic lineage). For example organ transplanted from pig to human, baboon to human.

Genetic basis of organ transplants :

Success of organ transplants (ie., Cornea, Kidney, Heart, Liver, Bone marrow) and skin grafts depends on a proper matching of histocompatibility antigens that occur in all cells of the body. Chromosome 6 of mouse contains a cluster of genes known as themajor histocompatibility complex (MHC), which in humans is called human leukocyte antigen (HLA) complex. The alleles of HLA genes determine the histocompatibility ie., the compatability between donor and recipient tissues in transplants.

Process of graft rejection : (Allograft)

When the graft or tissue involves two genetically distinct members of the same species, graft rejection occurs because the antigens of the graft and host being different, the immune response of the host rejects the graft. The graft dies, decays and is eliminated from the host. The host also reacts to the graft and shows the following symptoms. 1.Skin rashes, 2. Fluid accumula-tion in spleen and enlargement (Splenomegaly), 3. Emaciation (becoming thin), 4. Diarrhoea, 5. hepatomegaly, 6. Anaemia and general immune suppression, 7. Damage in bile ducts, 8. Increased bilirubin synthesis etc. Both cell mediated and humoral immune responses follow in rejection. Sensitized T cells (lymphocytes), macrophages, plasma cells are all involved in the primary or first set rejection. In the secondary or second set reaction, B cells (B lymphocytes) and their antibodies are involved. In the cell mediated reaction substances such as interleukin 1 (IL-1), Interleukin - 2 (IL-2) etc take part. The final lysis of the graft is achieved by lymphotoxins or TNF (Tumour necrosis factors) or proteolytic enzymes.

In clinical fields, graft rejection is prevented by : 1.Blood groups estimation (ABO and Rh) in the host, 2. Testing the presence of cytotoxic antibodies in the host serum, 3. Cross matching of tissues (Host Vs graft) prior to transplantation, 4. Giving immunosuppressive drugs like cyclosporin and steroids etc to the host, 5. Total lymphoid tissue irradiation etc.

In recent years, the cloning technology promises to bring solution to the problems of graft or tissue rejection in transplantation surgery. By stem cell technology and cloning of cells, organ culture is feasible. Organs cultured from the same embryo or individual are safe and valuable for transplantation surgery.