Type 12: Transplantation

Iatrogenic

(effects of medical treatment) such as blood transfusion or solid organ transplantation

Natural

Such as pregnancy in placental mammals

Transplantation

The introduction of biological material- organs,tissue,cells or fluids into an organism.

What is the main issue with transplanting tissue?

Most cells express polymorphic surface antigens encoded by MHC- variation between donor and recipient at MHC results in rejection.

Even if there is perfect match other “minor” antigens can be recognised by the immune system.

What are the 4 possible relationships between transplanted donor material and recipient:

1. Allogenic

2. Syngeneic

3. Autologous

4. Xenogeneic

Allogenic

Most common type of transplants

• Source: A different person (a human donor).

• Process: Stem cells are taken from a donor, who can be a related or unrelated match, and given to the recipient.

• Key Feature: The donor's cells are a close genetic match, but still different enough that there is a risk of the recipient's body rejecting the new cells, or the new cells attacking the recipient's body (graft-versus-host disease). 

Displays immunological memory

The immune system is primed by the allograft, upon the first encounter with antigen. When a recipient that has previously rejected a skin graft is regrafted with skin from the same donor the graft is rejected more rapidly, in a second-set reaction. Skin from a third-party donor grafted onto the same recipient would be rejected as for the first-set. The rapid second set rejection course can be transferred to normal or irradiated recipients by T cells from the original recipient. This shows that second-set rejection is caused by a memory immune response from clonally expanded and primed T cells specific for the donor skin. Memory T cells are produced alongside effector T cells in a primary immune response. Upon a second exposure to the same antigens memory T cells promote a more rapid, more effective response. This is because memory T cells to a specific antigen are more numerous than naïve T cells and they are more readily activated.

Syngeneic

• Source: An identical twin.

• Process: Stem cells are taken from the identical twin and transplanted into the patient.

• Key Feature: Also has no risk of rejection, as the donor and recipient are genetically identical. 

Autologous

• Source: The patient's own body.

• Process: Healthy stem cells are collected from the patient, stored, and then returned after high-dose chemotherapy or radiation to destroy cancer cells.

• Key Feature: No risk of the body rejecting the transplant because the cells are genetically identical to the patient.

Xenogeneic

• Source: A different species (e.g., an animal).

• Process: Tissues or organs from one species are transplanted into another.

• Key Feature: This type of transplant has a high risk of rejection and is more complex than others due to the significant genetic differences between species.

Rejection mechanisms

T-cells play the central role for rejection including NK cells which perform direct recognition of donor MHC or indirect recognition of an antigen presented by self MHC molecules.

Hyperacute rejection

Acute graft rejection

Direct recognition of allo MHC.

Indirect recognition

Properties of minor antigens

Chronic graft rejection

Particular transplant situations

1. Privileged sites

2. Vascularised solid organs

3. Haemopoietic stem cell transplants: Also called bone marrow transplants. There are three sources of stem cells:

1. Peripheral blood - enriched by cytokine administration – CD34+ pluripotent stem cells are mobilised by GM-CSF.

2. Bone marrow

3. Cord blood.

HLA matching