Recently a specific auto-antibody response has been found in patients with neuromyelitis optica, which is directed against the astrocytic water channel aquaporin 4. In experimental models these antibodies do not induce disease or lesions in the central nervous system, when present in the circulation of normal rats. However, when T-cell mediated inflammation is induced in such animals, circulating antibodies against aquaporin 4 can reach the brain and induce lesions in the central nervous system, which are closely similar to those, seen in patients with neuromyelitis optica.