Current thinking is that endometriosis is not an auto-immune disease, but the research is ongoing.

An auto-immune disease is where the body can’t tell the difference between ‘normal’ cells and ‘foreign’ ones, like a virus or an infection. The body’s defences start attacking its own normal cells, causing problems like pain and inflammation.

The current consensus is that in endometriosis cells are found in parts of the body where they don’t belong (so they are foreign to that part of the body) which makes the body’s reaction correct and understandable: it’s trying to fight off an unwelcome invader.