TRANSPLANT IMMUNITY Flashcards
(12 cards)
Define Transplantation
The process of taking cells, tissues or organs from an individual and placing it in a different individual or a different site in the same individual
What is a graft?
Transplanted cells , tissues or organs.
State and define the different type of transplants
Autograft - cells , tissues or organs transferred from one site to another in the same individual.
Isograft - cells, tissues or organs transferred between genetically similar individuals.
Allograft - cells, tissues or organs transferred between individuals who are not genetically similar
Xenograft - cells, tissues or organs transferred from one species to another
Give how graft rejection occurs regarding MHC molecules
In the adaptive response towards a graft, MHC mismatch may occur between the Donor cells and recipient’s T cells.
The graft is thus recognized as foreign, and an immune response is mounted against it.
There is also the effect of minor histocompatibility antigens that may trigger graft rejection.
T cells recognize alloantigens by two main pathways , which and which
Direct vs Indirect
Differentiate between direct and Indirect recognition of alloantigens by T cells
Direct involves T cells recognizing the foreign MHC molecules by the T cells without antigen processing by the host APCs. This triggers an immune attack against the alloantigens
Indirect pathway involves the T cells recognizing peptides derived from the graft that are presented by the host APCs using the host MHC.
Before Recognition of the alloantigen by either the direct and Indirect pathway , sensitization of T cells usually occur . Briefly explain.
Sensitization to the allograft usually occurs in the lymph node draining the graft. The donor APCs and recepient APCs containing the alloantigens migrate to the lymph node and sensitize the T cells by either the direct or indirect pathway.
After sensitization there is generation of effector T cells.
How do effector T cells mediate rejection
The effector T cells are the CD8+ and CD4+ T cells.
The CD8+ T cells generated from direct mediate rejection by CTL-mediated killing
The CD8+ generated from the Indirect pathway release cytokines causing inflammation and rejection.
The CD4+ T cells can activate B cells to plasma cells and release antibodies. They can alco release cytokines that activates other immune cells.
Describe hyperacute rejection
It occurs within minutes to hours after transplantation. It is usually due to prexisiting antibodies in the donor that may be due to previous exposure such as in pregnancy or blood transfusion. It is characterized by thrombotic occlusion when the pre existing antibodies bind to the endothelium.
Describe acute rejection
It occurs within weeks. It occurs due to alloreactive T cells mediating rejection by CTL-mediated killing and cytokine release . It is also due to antibodies against the endothelium.
Acute rejection is charactirized by parenchymal rejection and endothelial injury and occlusion.
Describe GVHD/GVHR
This is when the donor immune cells attack the recipients tissues. It is common in Hematopoietic stem cell transplant / Bone marrow transplants where the patients are immunocompromised after undergoing radiation/ chemotherapy for treatment of the leukemia.
It may be acute (within the first few months) or chronic (between months to years).
Give four tests done before transplantation to minimize rejection
1) Blood typing and ABO compatibility
2) HLA typing
3) Screening for preformed antibodies
4) Mixed lymphocyte reaction.
