The most common food allergens are nuts, dairy, gluten and soy. But some people are allergic to different fruits also. It takes a lot of work to avoid gluten and dairy, but it is worth it. Get tested for Celiac disease.
Most of the times the causative allergen remains unknown and thus fall with the terminology 'Chronic Idiopathic Urticaria'.However this is no reason to trying to identify a cause.