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Organ and tissue transplantation is an important treatment procedure. The tissue or organ transplanted is called a transplant or graft. The organism from which the transplant is taken is called a donor, while the individual on whom it is done is called a recipient.
Antigens present in the RBC are also found widely in other cells of the body. Apart from this, each body tissue has its own set of antigens. Therefore, when foreign cells are transplanted into the body of the recipient, immune reaction occurs.
Most of the experimental work related to transplants has been carried out in mice. In mice, the genes that are responsible for graft rejection are called histocompatibility genes or H genes. A large number of H loci exists throughout the mouse genome.
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The strongest locus is known as major histocompatibility complex (MHC). The major histocompatibility complex (MHC) is a series of genes that code for cell surface proteins which control the adaptive immune response. The system is called H2 in mice and HLA (human lymphocyte antigen) in humans.
Success of organ transplants and skin grafts requires a matching of about 150 antigens known as histocompatibility antigens or HLA that occur on all cells in the body. The test carried out to match the HLA antigens of the donor and the recipient is called tissue typing.
HLA antigens are present on antigen-presenting macrophages, dendritic cells and B-cells. The function of these proteins is to present fragments of antigens to T-cells. The receptor of T-cells can only recognise antigen fragments in complex with MHC proteins.
In each individual, however, only six antigens are present on the cell membranes. It is virtually impossible for any two individuals to have the same six antigens, except in the case of identical twins. In humans, the MHC is called the Human Leukocyte Antigen Complex (HLA). These genes are present on chromosome 6. The array of HLA alleles on one copy of chromosome 6 is known as a haplotype.
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The large number of alleles involved means no two individuals, even in a family, will have the same identical haplotype. Identical twins have a 100% HLA match. The best matches are going to occur within a family.
The preference order for transplants is identical twin > sibling > parent > unrelated donor. Chances of an unrelated donor matching the recipient range between 1 in 100,000-200,000. Matches across racial or ethnic lines are often more difficult. When HLA types are matched survival of transplanted organs dramatically increases.
Types of Transplants:
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Depending on the donor, given below are the different types of transplants.
a. Autograft:
A transplant of a tissue or whole organ from one part of an animal to another part of the same animal is called autograft.
b. Isograft:
When the transplantation is between one identical twin and the other twin.
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c. Allograft:
When the transplantation is between two organisms belonging to the same species.
d. Xenograft:
When the transplantation is between two organisms belonging to two different species.
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Several methods are currently in use to prevent graft rejection:
a. Tissue typing is a necessary precaution to be taken before any surgery is conducted.
b. Exposure of the bone marrow and lymph tissues to X rays inhibits production of white blood cells and slows down rejection. But there is a risk of infection during treatment and sometimes, unpleasant side effects occur.
c. Immuno suppression is the use of chemicals to inhibit the activity of the entire immune system. But this also increases risk of infection and patients are found to be more prone to cancer. Imuran is a drug currently used to suppress rejection of organ transplantation. This drug blocks the synthesis of DNA necessary for the multiplication of the lymphocytes and has to be taken throughout the life of the individual.
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d. Monoclonal antibodies recognise and destroy only the killer T-cells that are responsible for rejection.
This way the rest of the immune system would continue to function normally. Drugs and naturally occurring substances that can alter the activity of the immune system are known as immune modulators. It includes vaccines, interferons and immunosuppressive drugs.
