AQP4 and neuromyelitis optica: Also, T cells specific for other CNS antigens, for example, myelin basic protein or S100β, can open the blood–brain barrier for the entry of NMO‐IgG.6, 7 This is a very important point, as in the immune repertoire of NMOSD patients, expanded populations of AQP4‐ and proteolipid protein‐specific CD4+ T cells have been described, and other expanded populations of CNS antigen‐specific T cells might just not have been searched for.10, 11, 12