Tree nut allergy affects approximately 1.4-2.3%, depending on the study methodology, patient age, and region in which the study was conducted. Tree nut allergy can be severe, and tends to persist throughout life. Tree nut is not a singular allergen like the other allergens-we use the term "tree nut" to refer to these items, but in essence it can refer to one nut, eight nuts, or per the US Food and Drug Agency classification, on the order of 19 items. Older, more conservative practice is to treat one nut as "all nuts", and instruct avoidance of all tree nuts where there is a clinical allergy proven to just one. This is often supported by high rates of co-sensitization, which is high among tree nuts because of structural similarity, and fueled concerns of cross-contamination, proper identification, or labeling confusion.