Immunohistochemical study of lymphocytes in rat pineal gland: Selective accumulation of T lymphocytes

Morphologic and immunohistochemical studies were performed to examine the existence of lymphocytes in the brain of rats. Special attention was paid to the time course of the appearance of lymphocytes in and around the pineal gland. Rabbit anti‐rat T cell and anti‐rat immunoglobulin sera were used for identification of T and B cells in tissue sections. Immunoperoxidase and immunofluorescence techniques were employed to identify cells reacting with anti‐T and anti‐immunoglobulin sera. No lymphocytes were found in the brain of rats until 20 days after birth. Small clusters of lymphocytes appeared in the pineal region by 30 days of age, after which they gradually increased in number, forming massive clusters in the pineal region by 120 days. Along with an increase in the number of lymphocytic cells, there was a gradual increase of cells reacting with anti‐T cell serum. These T cells were only a minority of pineal lymphocytes in younger animals, but 90% or more cells were stained by anti‐T cell serum at 120 days after birth. The remaining cells did not react with anti‐immunoglobulin sera either. These findings suggest that the gradual increase of T lymphocytes in the rat pineal region is a simple reflection of the normal course of maturation of T cells, and the pineal gland in the rat may have some role in immune responses within the brain.