Is Plastibo in Your Head?

i think placibo is all the mechanisms of medicine that we don't understand. We are able to observe their effect, but we don’t understand it, so we assume it's in the head - which i thin is false. Does belief have a particular impact on your physiology, sure,. But that doesn't mean it's all in your head, right? If you can observe something in your physiology, that means, by definition, it's not in your head.