Blood samples were taken from 228 people in a UK-wide trial.
The finding is controversial, but opens up a new way of treating patients.
Sarah Galloway, 25, from Gateshead, is now recovering after her immune system turned on her.
She was in the final year of her chemistry degree when things started to go wrong.
"I deteriorated in a matter of days," she says.
"I hallucinated that my body has morphed spiders' legs or rabbit ears, I've seen them there, I've felt them there.
"I get strange ideas in my head that someone is trying to kill me or I have to kill someone, and then a lot of it has resulted in self-harm."
Sarah was rapidly sectioned by doctors, given a diagnosis of bipolar disorder and treated with anti-psychotic medication.
However, a chance blood test turned up something unusual in her immune system.
Antibodies should protect the body, but instead Sarah's were attacking the surface of her brain's cells and disrupting their function.
That transformed Sarah's treatment, and she was given drugs to suppress her immune system.
She even had troublesome antibodies filtered out of her blood.