Allergic reactions are the primary cause of hives. It can be difficult to determine the specific inciting factors, but there are numerous stimuli that can cause them to appear. Food products, plant contact or ingested medications can cause hives. Physiologically, they occur when there is a large histamine release in the body from specialized blood cells called mast cells. The histamines cause fluid to leak around the blood vessels in the area they appear leading to the skin changes that can be easily seen.