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?
At one of my previous companies that raised antibodies to proteins and post-translational modifications, we found that 6-8 amino acids was generally the smallest peptide length required for an epitope. Anything much smaller than that won't induce the cross-linking or conformational changes required for signal transduction and an allergic response. As WYSIWYG notes, allergies to small molecules/atoms are formed via hapten-carrier interactions. These can be covalent, as in the nickel-transferrin example, or as a result of denaturation/chemical modification, as likely happens with people who are allergic to things like isopropanol or bleach.
I very much doubt that there can be such a thing as a glucose allergy, as it is so ubiquitous in the body that such antibodies/T-cells would be negatively selected long before they could do any damage.
Small molecules do not have antigenic properties but they can elicit immune response by binding to a "carrier" protein. The small molecule is called a Hapten. An epitope forms at the protein-hapten binding interface.
Nickel acts like a hapten and elicits immune response by binding to proteins like transferrin, albumin etc.
Other metals also cause allergy in a similar way.
