Mustard allergy is reported relatively rarely but is a growing concern, at least in Europe. According to a 2003 editorial in the European Journal of Allergy and Clinical Immunology, mustard was the fourth most prevalent food allergen in children, behind eggs, peanuts, and milk.Boston Globe has recently published a case of a 3-year-old girl who is allergic to mustard -- read more on AllergyNotes.
Mind map of food allergy. Source: Food Allergy: A Short Review, AllergyCases.org.
Image source: Wikipedia, GNU Free Documentation License.