My wife is severely allergic to both tree nuts and peanuts (as in full on anaphylaxis, hello epipen and ER). We both find this curious, since peanuts are legumes and not nuts at all. Informally we've heard there is about a 10% overlap in these allergies. Does anyone know why that might be? I know this seems more like a medical than cooking question, but I think it has a relevance to cooking as it helps us understand both the composition of these foods and how we may best as cooks help to avoid poisoning people we cook for.
So from this - http://content.karger.com/produktedb/produkte.asp?typ=pdf&file=IAA2003131004234 - it sounds like they both share IgE-reactive (http://en.wikipedia.org/wiki/Immunoglobulin_E) epitopes.