Peter Jones once treated a patient with strikingly vivid hallucinations of animals. During ward visits, the patient would stroke and interact with the critters “in as real a way as he was with me,” says Jones, an academic psychiatrist at the University of Cambridge in England.

Antipsychotic drugs did nothing to dissolve these Snow White-esque visions. But when the patient received plasma exchange –– a treatment normally reserved for a misbehaving immune system –– his hallucinations dissipated.

Clearly, this was not a textbook case of schizophrenia. Rather, the patient’s symptoms stemmed from autoimmune encephalitis, a rare form of brain inflammation that can, in some cases, cause hallucinations and other symptoms of psychosis. When these patients are treated with drugs that tamp down the immune system, the psychotic symptoms often disappear.