EXPERIMENTAL studies in the past have suggested that the brain is a "privileged site" for homografts. The brain lacks a lymph node drainage system. Therefore, parathyroid homografts were experimentally implanted into the brains of dogs and rabbits to assess the significance of the brain as a "privileged site" and also the significance of regional lymph nodes in the homograft reaction. Microscopic studies of the implants and serum calcium determinations were criteria used in evaluating the status of the implants.
Permanently viable homografts of parathyroid tissue into brain were not achieved in these experiments. Parathyroid autografts did survive. Homografts were uniformly rejected within several weeks. Fat cells seemed to survive for longer periods in some animals and served to make the implantation site.
These experiments indicate that the brain is not sufficiently "privileged" to be of value as a homograft site in these species. Although lymph nodes and certainly lymphocytes appear
BLOCK MA, TWOREK EJ, MILLER JM. Parathyroid Homografts in Brain TissueExperimental Studies. Arch Surg. 1966;92(5):778–784. doi:10.1001/archsurg.1966.01320230126023