system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Are there certain foods you can't be allergic to?

I've never heard of anyone being allergic to oil, yeast, or vinegar, are those things you can't biologically be allergic to? I'm specifically wondering about ingestion allergies, not skin allergies. I posted this a while ago and was told it is possible to have an allergic reaction when water touches your skin, I googled it and couldn't find anything about an allergy when ingesting it, so I'm also wondering if anyone can be allergic to water when ingested, if not then why? Is there anything else you can't be allergic to when you ingest and why can't you be allergic to it?
gpt
(EDIT: The vast, vast majority of) Allergies are triggered by an inappropriate immune system response to specific proteins.

So, one cannot (EDIT: In the vast, vast majority of cases)  be allergic to pure water, pure salt, pure sugar, pure fat, pure sand, oxygen, etc.

Naturally, nothing handled by humans is going to be pure; there are always going to be impurities.  If you have an allergy, you have a threshold (however low) below which your allergies won't be triggered.

This is why most people with peanut allergies can consume *refined* peanut oil without problems -- it's all fat (or, close enough to all fat for all intents and purposes).  But fancier, cold-pressed peanut oils cause problems because they leave a lot of the protein behind.