What can cause hives besides an allergic reaction?

Releasing histamine: Hives are due to a release of histamine in the skin, commonly due to allergy to foods, medicines, viruses, molds, animals, pollens, etc... Some people release histamine and get hives when exposed to cold temperatures, cold water, sunshine, stressful situations, etc... Other people with hives may have internal medical conditions such as chemical imbalances, intestinal parasites, etc...

Hives: Foods, infections (usually post-viral), autoimmune disease, hormones, medications, lymphomas (very rare).