All individuals carry a unique pattern of genes in their body. Some of these genes are expressed on each cell in our body. On the surface of each of our cells is a group of proteins called the "HLA complex." This complex is practically unique for each individual, except for identical twins who share the same genes. This complex acts as a unique molecular identification for every human being.
If we simply put cells from one individual in to the body of another, the recipient’s immune system (a highly efficient protective system of blood and lymph cells) can recognize these outside cells as "foreign" and attack and kill them. This results in a potentially fatal condition called "graft-versus-host disease" (GVHD). Therefore, it is challenging to replace a dysfunctional organ or defective bone marrow easily.
If the person needing a transplant has an identical twin, there’s nothing to worry about. Identical twins have the same genes and an identical HLA complex. For others, it is vitally important to find a donor whose HLA matches as closely as possible to their own. A closely matched HLA reduces the chances of transplant rejection as well as the chances of developing GVHD.
Source:
Cancer: Principles and Practice of Oncology 7th Edition. Editors: VT DeVita, S Hellman and SA Rosenberg. Published by Lippincott Williams and Wilkins, 2005.
