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.
Both native and recombinant nut allergens have been identified and characterized and, for some, the IgE-reactive epitopes described. Some allergens, such as lipid transfer proteins, profilins, and members of the Bet v 1-related family, represent minor constituents in tree nuts. These allergens are frequently cross-reactive with other food and pollen homologues, and are considered panallergens. Others, such as legumins, vicilins, and 2S albumins, represent major seed storage protein constituents of the nuts.