Immunologically Privileged Tissue
Due to the well-known concept of allograft rejection, it is common clinical practice for donors and recipients to be screened and matched prior to organ transplantation. For example, the major histocompatibility complex (MHC) of the donor and recipient must be identical; the recipient must have no infection and must not have hypertension. In some instances, there are issues in the human body that do not necessarily require a match. Such tissues can be transplanted regardless of the antigenic nature of the donor and recipient without rejection. They are known as immunologically privileged tissues.
An outstanding example of Advanced Global Research immunologically privileged tissues is the cornea of the eye. The first triumphant corneal transplant was accomplished on December 7, 1905 by Dr. Zirm Eduard. Since then, there has been no organ transplant that has had similar success as a corneal transplant. The cornea has a highly avascular structure, so it does not allow alloantigens to access regional lymphocytic tissue. Furthermore, corneal antigens are masked and therefore cannot come into contact with the host's immune cells. These two mechanisms that prevent the receptor from activating an immune response prevent rejection.
In addition to the cornea, there are other sites in the body where allografts are rarely rejected. They can also be classified as immunologically privileged sites, although they have been overcome by corneal transplantation. They include the uterus, testicles, brain, and renal tubule. There are several mechanisms that allow these sites to have immunological privileges:
- The tissues lack lymphatic drainage. This is a limiting factor because it prevents immune cells from flowing to the regional site.
- The sites have fas ligands. These are death receptors on the surface of the tissues that kill fas-expressing lymphoid cells, increasing immune privilege.
- These immunologically privileged sites produce immunosuppressive cytokines that depress the host's immune system. A good example of suppressors is TGF-β, a proteinase substance that controls cell differentiation in many cells.
Faced with this problem, researchers are trying another method: the transplantation of stem cells extracted from the umbilical cord of newborns. These immature cells are capable of regenerating the bone marrow, and it is best when HLA (human leukocyte antigen) compatibility is observed. These immature cells decrease the phenomenon of graft rejection. Furthermore, the proliferation capacity is superior to that of adult bone marrow cells.
Many therapeutic advances have been observed in children with leukemia in the field of umbilical cord blood transplantation. In adults, this type of transplant is also possible in some cases.
Transplant, which does not require histocompatibility, makes it possible to shorten waiting time, which can be very important in an emergency (for example, acute leukemias that require immediate intervention) because marrow samples are often not available with compatible bone.
Studies continue in this field, as it seems that perfect compatibility is not essential for a successful transplant. This offers great hope, especially for leukemia patients who expect a compatible marrow.