Histocompatibility refers to the compatibility of tissues from different individuals, particularly in the context of organ transplantation. It is crucial for ensuring that the recipient's immune system does not reject the transplanted organ. This compatibility is primarily determined by the presence of specific proteins known as Major Histocompatibility Complex (MHC) molecules on the surface of cells. When a donor organ is transplanted, the recipient's immune system recognizes the MHC molecules. If these molecules are similar between the donor and recipient, the risk of rejection decreases. Testing for histocompatibility is essential before transplants to improve success rates and patient outcomes.