A11 Transplant

Topic

Transplantation replaces an organ or tissue that has ceased to function with one from a donor. Its primary biological challenge is rejection: the recipient's immune system recognizes the donor's histocompatibility antigens as foreign and mounts a response against the graft.

Antigen-presenting cells associated with the graft expose donor antigens to the recipient's T lymphocytes. Lymphocytes capable of recognizing these antigens become activated and proliferate, forming a population of alloreactive T lymphocytes that can progressively attack the transplanted organ. Immunosuppression acts on this process by reducing both the initial activation of these lymphocytes and their subsequent expansion, thereby limiting immunological damage to the graft.

Compatibility between donor and recipient histocompatibility antigens is a key determinant of long-term rejection risk. Three main groups are considered, each inherited as two copies—one from each parent. The greater the number of differences among the six copies between donor and recipient, the higher the risk of chronic rejection. This degree of incompatibility also correlates with graft survival: in kidney transplantation, complete compatibility is associated with a 60–70% ten-year survival rate, whereas maximal incompatibility results in approximately 40–50% survival.

Tacrolimus is one of the primary immunosuppressants used in solid organ transplantation. It blocks a pathway required for T lymphocytes to produce signals that stimulate their proliferation, thereby reducing the expansion of alloreactive cells. Because there is a narrow margin between insufficient immunosuppression and potentially toxic levels, the minimum blood concentration of tacrolimus is regularly measured immediately prior to the next dose.

The target concentration depends on the specific organ and the time elapsed since the transplant. Levels are typically kept higher during the first few months—when the risk of rejection is greatest—and are subsequently reduced. The dosage required to achieve a specific concentration varies widely among patients due to factors such as differences in the activity of the liver enzymes that metabolize the drug. Consequently, treatment must be individualized through the monitoring of blood levels.

Another decisive factor is the duration for which the organ remains outside the donor's body in cold storage. During this period of cold ischemia, cellular and mitochondrial damage accumulates, potentially impairing the graft's initial function and long-term survival. Each organ has a different tolerance level: kidneys can be preserved for approximately 24–36 hours, livers for 12–15 hours, and hearts for only 4–6 hours. Specialized preservation solutions and cold perfusion systems aim to slow the rate of this damage and extend the time available for transport and transplantation.

Following the procedure, a constant balance must be maintained between two opposing risks. Insufficient immunosuppression increases the risk of rejection, whereas excessive immunosuppression compromises the body's ability to fight opportunistic infections and control certain tumors. Patient monitoring must therefore involve continuous adjustment of the treatment to preserve graft function without excessively weakening the recipient's immune defenses.

Graft survival models incorporate factors such as compatibility, the intensity of immunosuppression, and initial organ function to estimate the risk of graft loss and the likelihood of long-term function. According to these models, grafts from living donors with good compatibility can achieve ten-year survival rates of 65–75%, whereas grafts from deceased donors with high levels of incompatibility show rates of approximately 45–55%. Overall, transplant success depends on controlling the immune response against the graft, carefully selecting and monitoring immunosuppression, minimizing damage during preservation, and continuously balancing the risk of rejection against the risk of infection. Clinical practice integrates these factors—from organ selection and preservation to long-term follow-up—with the goal of maintaining graft function for as long as possible.

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