I Have A Good Friend Who Has To Go To

I have a good friend who has to go to the hospital for what appears to be severe allergy to something, but nobody can figure out what. She has these episodes every week or two sometimes she has to stay at the hospital for several days. This problem has just come up in the last year. I am worried that something might be going on with her immune system, because the episodes are random and there are no known allergy triggers. I suggested maybe seeing an immunologist. Is there any condition that might have similar symptoms?