Transplant immunology: Types of graft, and transplant rejection

Transplantation immunology:

The process of transfer of cells, tissues, or organs from one location to another with a motive of either repairing or replacing damaged or diseased organs and tissues is defined as transplantation.
In case of failure of organ system or damage, it can be replaced by the healthy organ or tissue donated by donor.
Transplantation is life savior procedure and is employed only when other treatment options fail.
However, the immune system is the one to play a significant role in transplantation.
The successful transplantation is all dependent on the complex mechanisms of immunity.

The following terms indicate different types of transplants:

In this type of graft, the tissue is transferred from one body site to another in the same individual.
For example, use of healthy blood vessels to replace blocked coronary arteries.

2. Isograft:

The transfer of tissue between genetically identical individuals is termed as isograft.
For Example the transfer of kidney from one identical twin to the other.

3. Allograft:

In this type of graft, the tissue transfer takes place between genetically non-identical members of same species.
Example: skin transplant between two individuals of different genotype.

4. Xenograft:

The transfer of tissue or organ between different species is termed as xenograft.
For example: the graft of monkey’s heart into a human.

5. ABO incompatible transplant:

ABO is common term for blood group, which differs among individuals.
The key strategy applied for the minimization of transplant rejection is the matching of blood group between donor and recipient.
However, the compatibility is always not required for transplantations. For instance, ABO transplants can be carried out in children with immature immune systems with minimized risk of transplant rejection.

6. Stem cell transplant:

Stem cells are capable of giving rise to indefinitely more cells of same type, and from which different other cells arise by differentiation.
Hematopoietic stem cell transplants are done to replace damaged or worn out blood cells. Also, it is employed to treat various forms of cancer, eg. Leukemia.
These stem cells can be obtained either from cord blood or from bone marrow.

Immune system works to recognize the foreign microbes or foreign threats and destroys them creating a barrier for the transplantation.
When the immune system identifies the transplant as a foreign, it initiates a response that ultimately degrades the transplanted organ or tissue.
This rejection caused by immune system in case of transplantation is termed as transplant rejection.
The organ or tissue to be transplanted is termed as graft.
Depending on the type of graft being transplanted and the genetic variance between the donor and the recipient, the intensity of the immune response varies accordingly.
Thus, to prevent graft rejection, both the donor and the recipient are carefully matched for immune compatibility before the transplantation.
The immune system can be manipulated for long term survival of the graft which ensures successful transplantation.

When the immune system confronts a foreign organism, it prepares for attack against it in order to protect the body from infection.
It is must that the immune system should be able to distinguish between own healthy cells/tissues and foreign substances.
Foreign invaders appear in form of small molecules termed as antigens.
These molecules, when presented to the immune system, triggers the immune response.
It stimulates the production of antibodies specific to those antigens and amplifies the immune response.
The group of genes that encodes proteins which identifies foreign agents to the immune system is termed as the Human Leukocyte Antigen (HLA) complex.
These proteins act as ‘self-markers’ as they convey immune system not to trigger a response and are present on the surface of every cells.
On the basis of genetic makeup, each individual shall have their own specific set of HLA proteins.
Any cell unable to display HLA proteins will be identified as non-self by the immune system and will be further responded.

The term histocompatibility is used to represent the degree of similarity between the HLA genes of the donor and the recipient.
The compatibility between the donor and the recipient depends on the similarity of genetic makeup between them.
However, there will always be some extent of rejection even if the donor and recipient are genetically identical.
Non-self HLA proteins, other surface proteins on the donor graft can also be recognized as a foreign antigen and forbid an immune response.
In some cases, a patient encounter ‘graft versus host reaction’ where mature immune cells already available in the donor graft attacks the healthy cells of the recipient.

1) Hyperacute rejection:

The presence of pre-existing antibodies of the recipient, that match the foreign antigens of the donor, triggers an immune response against the transplant and results in this type of rejection.
These antibodies could have been produced due to result of previous blood tranfusions, previous transplantations or multiple pregnancies.
This takes place within minutes or hours after a transplantation.
The blood clotting takes place, when antibodies react with cells in the blood vessels of the graft, which will prevent blood supply from the graft yielding an immediate rejection of the transplant.

2. Acute rejection:

This takes place within the first 6 months after transplantation.
Exception to identical twins, there prevails some degree of acute rejections in all transplantations.
There is a high risk for the first 3 months for recipients, however rejection can still take place at a later stage.
After the detection of non-self antigens, the formation of antibodies causes acute rejection.
By suppressing the immune system, acute rejection can be treated to some extent and the permanent damage to the graft can be avoided in some cases.

3. Chronic rejection:

Recurrence of acute rejection can definitely lead to chronic rejection of the graft resulting the failure of transplant.
The exhibition of chronic rejection takes place as scarring of the tissue or organ which can occur for months to years after acute rejection has subsided.
There is no cure for chronic rejection except the removal of graft till date.