5 Blood-cerebrospinal fluid barrier and Virchow-Robin space not only act as a natural line of defense,but also prevent the inflammation to penetrate to the parenchyma 6 There is no resident lymphoid tissues,the lymphocytes are all conveyed by the blood circulation ⑤ Blood-cerebrospinal fluid barrier and Virchow-Robin space not only act as a natural line of defense, but also prevent the inflammation to penetrate to the parenchyma ⑥ There is no resident lymphoid tissues, the lymphocytes are all conveyed by the blood circulation