Hives and Angioedema: Hives is a sudden allergic skin reaction. Hives are raised reddish bumps that cause severe itching. Angioedema is similar to hives but the welts are larger and involve a deeper layer in the skin. Allergies, insect stings and bites, stress, exercise, and infections are common causes of hives and angioedema. In mild cases, no treatment is needed. Severe reactions should be treated by a health-care provider.