Incompatible matching of donor and recipient tissues is one of the major challenges of organ transplantation. Incompatible matching of MHC molecules between the donor and the recipient usually results in activation of the immune responses. In this case, activated immune system attacks to the transplanted organ and tries to destroy it. Adaptive immunity is thought to be the main response exerted to transplanted tissue or organ. Adaptive immunity is mediated via B and T cells. T cells recognize antigens displayed on MHC molecules, and they trigger an immune response if the antigen on MHC molecule is recognized as foreign. If MHC molecules with antigens on transplanted donor tissue are recognized as foreign, T cells are activated. Activation of T cells lead to further activation of other immune responses such as expression of cytokines and chemokines, which in turn recruits more immune system…show more content… Organs have different properties such as the amount of blood vessels it contains or the number of immune system cells that have access to the organ. As mentioned before organs like eye and brain have less immune system cells that have access to them, therefore most transplantations do not result in rejection for those organs. However, MHC molecules of the highly vascular donor organs have better chances to encounter an immune system element and trigger immune system. Although exact mechanism that leads to organ specific responses are not known, it is observed that kidney transplants are tolerated more than heart transplants. Kidney is said to be tolerance-prone whereas heart is said to be tolerance resistant. Another interesting point is that kidney allografts can grant unresponsiveness for co-transplanted tolerance resistant organs such as heart. As can be seen from the figure below, heart transplantations have higher success rate when co-transplanted with
