While I can't help with the Xolair portion of your question, I do have some not inconsiderable experience with a teen who has a very low threshold.
I was wondering if anyone on this board deals with a contact allergy and how you do it.
Well, you learn to play it by ear, obviously-- there are things that we have our DD (now15) avoid, and I couldn't probably even explain WHY we do those things, because it's not always clear why the risk exists at a particular activity/location, but just that it DOES.
For example, I know that about 10-25% of the time, DD is going to have contact reactivity if her allergen is around. This is allergen dependent, of course, but peanuts and treenuts are a definite culprit there.
We've also learned over the years that CONTACT reactions aren't especially dangerous-- just irritating-- but it's what they signify that makes them scary and worth avoiding. DD's ingestion/internal exposure threshold is SO low that we know that any contact reaction is an indication that a setting is MASSIVELY dangerous to remain in, because an amount large enough to elicit cutaneous symptoms with contact is more than enough to cause anaphylaxis if ingested or inhaled.
Other kind of ritualized things that we do:
a) handwashing-handwashing-handwashing-- every time we re-enter our home, and definitely every time before eating or handling food-- ANY food.
b) we carry clorox wipes everywhere. Purse packs are awesome. Some people use baby wipes, which are easier on the skin, but we can't b/c of my dd's aloe sensitivity.
c) DD is profoundly aware of where SHE puts her hands-- she avoids common touch-surfaces like stair railings, doorknobs, etc. with her bare skin. Period. This particular behavior avoids an enormous number of problems. It also means, however, that you can bring residue home on jacket sleeves (she pulls her sleeve over her hands to open doors, etc.)-- so be cautious about that in a setting that is LIKELY to be contaminated.