Hello and Welcome to ‘Ask A Doctor’ service. I have reviewed your query and here is my advice. Well, if the welt like rashes appeared after eating Pizza that is the newly and the first time ever eaten food then any of the ingredients of the pizza might be a cause for the allergic reaction. So my suggestion is that you should first of all get it confirmed by getting an Allergy test done like food challenge or skin prick test done to rule out the allergen. Once the allergen is ruled out then the best way to prevent similar reactions is to avoid getting exposed to allergen or eating such foods with the allergic ingredient if food ingredient is the cause. For now she can take anti allergics like Benadryl or Levocetrizine to reduce the allergy symptoms. Also she can apply cool compresses or ice packs over the welts.. Applying benadryl cream can also relieve the irritated and itchy skin. Hope I have answered your query. Let me know if I can assist you further. Regards, Dr. Honey Arora