Transplantation barriers

Given the extent of the natural polymorphism in a population, the donor and the recipient will always differ for allelic variants of numerous proteins (except for monozygote twins). Such differences will represent transplantation barriers when recognized by the immune system.

Alloantigenic structures

Blood groups
Blood groups represent important transplantation barriers when blood is transfused because incompatible ABO blood group antigens of the donor are recognized by the recipient’s natural antibodies. Blood group antigens are also expressed on vascular endothelial cells of the organs. Hence, matching of blood groups is essential in organ transplantation (even if no blood is transfused!) because the recipient's natural antibodies may recognize the antigens on the graft and cause its rejection.Read more ...
Major histocompatibility antigens (MHC)
The way blood groups antigens are recognized is quite different from the recognition of the other type of transplantation barriers, the “Histocompatibility Antigens” (also known as transplantation antigens). They were first identified in transplantation experiments in mice and because at that time, nothing was known about the nature of the antigens, the term “Histocompatibility Antigens” that stands for “molecules playing a role in tissue compatibility” (Greek: histo meaning tissue) simply refers to their role in transplantation reactions. A group of genes that encoded for antigens that caused a particularly fast rejection of histoincompatible skin grafts was then designated as the major histocompatibility complex (MHC). Read more...
minor histocompatibility antigens (mHC)
Other histocompatibility antigens caused a much slower skin rejection and were consequently designated as minor histocompatibility antigens (mHC). Read more...

Role and recognition of the alloantigenic structures in transplantations

Which type of alloantigenic structure represents the strongest (+++) transplantation barrier depends on the type of transplantation:

	Organ Transplantation	HSC Transplantation	Blood Transfusion
Blood groups	+++	(-)	+++
MHC	++	+++	(+)
mHC	(+)	++	-
Therefore, the criteria that makes a donor the most compatible are different for organ transplantation, for HSC transplantation or for blood transfusion.

The response to histoincompatible tissues (incompatible MHC and/or mHC) was designated as allorecognition. Histocompatibility antigens can be recognized by (allo)antibodies and by T cells. Allorecognition at the basis of a rejection of an organ have many points in common with the allorecognition causing Graft versus Host Disease. The response is initiated when alloantigens expressed by the stimulator are recognized by so called "alloreactive" T cells of the responder. Read more...

Which histocompatibility antigen is recognized and whether the antibody or T cell response prevails depends on the type of transplantation as well as of the degree of matching betweeen stimulator and responder. Read more...

Xenotransplantantion

Xenotransplantation involves transplanting living cells, tissues or organs between species, and in particular, from animals to humans. It is being considered because worldwide the demand for human cell, tissue and organ donation far exceeds the supply. The main risk for a person receiving animal transplant therapy is that the transplant might not function properly. However, there is also a risk that one of the wide range of viral, bacterial and other infections known to occur in the source animals will infect the transplant recipient. Read more...