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MHC molecules are essential for distinguishing 'self' and 'non-self' cells. The immune system recognizes foreign MHC molecules on transplanted tissue and can trigger a cytotoxic T cells mediated immune response, leading to transplant rejection. Each individual has a unique set of MHC molecules due to gene polymorphism and multiple alleles, making donor-recipient matching critical for transplant success.

MHC polymorphism refers to the presence of multiple alleles for each MHC locus, creating vast genetic variation among individuals. Combined with polygeny—the presence of multiple MHC proteins on cells—this polymorphism makes each person's MHC profile unique. This genetic diversity is why finding a suitable donor match is extremely challenging and often requires searching worldwide databases.

A successful transplant typically requires a match between at least 3–4 MHC molecules between donor and recipient. More matches are associated with increased success rates and reduced risk of rejection. However, achieving even this minimum level of matching is difficult due to the extensive polymorphisms in MHC molecules across the population.

Tissue transplantation treats end-stage organ failure including heart, liver, and kidney failure. Bone marrow transplants successfully address Severe Combined Immunodeficiency and leukemia. However, bone marrow transplants can cause graft-versus-host disease, where transplanted cells attack recipient tissues, leading to multiple organ damage and requiring careful management.

Graft-versus-host disease occurs when transplanted bone marrow cells attack the recipient's own tissues, causing damage to multiple organs. This complication represents a significant challenge in bone marrow transplantation. While efforts continue to moderate its effects, it remains an important consideration when evaluating the risks and benefits of bone marrow transplants for patients.

Finding a donor match is challenging because MHC molecules exhibit both polygeny and polymorphism, creating unique profiles for each individual. The extensive genetic variation in MHC loci means compatible donors are rare. Searches often extend to worldwide databases to locate unrelated donors whose MHC molecules sufficiently match the recipient's profile.

When donor and recipient MHC molecules mismatch, the immune system recognizes the transplanted tissue as foreign. This triggers an adaptive immune response, particularly a cytotoxic T-cell response that attacks and destroys the transplanted tissue. This immune rejection is the primary barrier to successful transplantation and is why MHC matching is so critical.