Final answer:
Each individual has a unique set of MHC molecules due to high polymorphism at the MHC locus, which is essential for the immune system's ability to differentiate between foreign and own cells. In tissue transplantation, compatibility largely depends on matching these MHC molecules, and the extensive polymorphism makes finding non-related donor matches challenging.
Step-by-step explanation:
Understanding MHC Polymorphism in Tissue Transplantation
Due to the high polymorphism of the MHC locus, each individual has a unique set of MHC molecules. The major histocompatibility complex (MHC) is crucial for the immune system to recognize foreign cells versus the body’s own cells. The MHC consists of a collection of genes coding for glycoproteins expressed on the surface of all nucleated cells. There are MHC polygeny and MHC polymorphism; polygeny refers to the multiple MHC genes, and polymorphism refers to the numerous alleles that exist for each MHC gene locus, contributing to the immune system's ability to respond to innumerable possible antigens.
In humans, the three highly polymorphic MHC class I genes are HLA-A, HLA-B, and HLA-C, and there are also three MHC class II molecules called DP, DQ, and DR. These molecules play a significant role in determining compatibility for organ transplants, and a successful transplant usually requires a match of several of these molecules. Due to the extensive polymorphism, a non-related donor match is often found through a global database, although the system is unable to meet the needs of all patients. Matching is critical because differences in these molecules between donor and recipient can lead to transplant rejection.
Monocytes, which are precursors to macrophages and dendritic cells found in the blood, are also involved in the immune response, including transplant rejection. Immunosuppressive drugs like cyclosporine A have improved transplant success rates but have not eliminated the need for precise MHC matching to minimize rejection risks.