immunology - What is the smallest molecule that can present as an antigen to the immune system in the context of allergies?

People often claim, in a colloquial sense, that they are "allergic to everything".

Is it possible to have a full-fledged IgE mediated allergic response to very small molecules? I was always under the impression that the smallest antigen was a oligopeptide, but is it possible for someone to be allergic to something like isopropanol, glucose, or another small organic molecule?

How about allergies to metals, as I know those are fairly common, would they be mediated through a similar mechanism?