NMO has only been relatively recently described and is fairly rare. Most NMO patients' immune systems produce abnormal antibodies against aquaporin-4 (AQP4), which is found in certain cells in the central nervous system. When these AQP4 antibodies bind to AQP4, they trigger a cascade of events involving the immune system which eventually leads to damage to the nervous system. This ultimately leads to disability, some of which is permanent.