Sulphites are one of the most common causes of food related allergic reactions and can cause extreme problems as the body over-reacts to their presence.
They are in seafood (especially shellfish), mushrooms, dried fruit, canned fruit, prepared potato products, vinegars, soy products - tons of things - in fact even in the packaging, as they are present in cellophane....
The reaction is not a true food allergy, but like phenylalanine, the presence of it must be noted to warn suffereres of conditions affected by it that they should avoid consumption.
Hope that helps..