A recent article in the Journal of Allergy and Clinical Immunology provided an update on our understanding of cross reactivity. 

First by way of definition an epitope is a discreet (usually small) portion of a molecule that is the binding target of an antibody.  In the case of allergic problems, the antibody is IgE.  By way of example think of distinguishing features that help you identify a car:  the Mercedes Star and the Dodge Ram. 

Allergy is directed at this epitope, not at the very large complete molecule.  As it turns out certain epitopes are found on both foods and airborne allergens.  The most common examples are crustaceans and dust mites, tree nuts and birch pollen, wheat and grass pollen.  The cross reactivity can be a two-way street where exposure to a food worsens an airborne allergy or vice versa.  Also, allergy shots for the airborne allergen can actually reduce the food allergy by desensitization reactivity to the shared epitope.  What will be very interesting to find out is whether desensitization to foods will help airborne allergy.  Food desensitization is still in its infancy with peanut desensitization being the main inroad in this regard.  But many academic centers have ongoing research to develop therapies for other common food allergens; milk, egg, wheat, soy and corn.  Stay tuned.