Allergic reactions can happen for many reasons, including such things as chemicals in certain foods, sunlight exposure, medications, or insect stings.

There are several different types of hives, including:

• Acute Urticaria – These types of hives last less than six weeks and are usually caused by certain foods, medications, infections, insect bites and internal diseases. The most common foods that cause hives are nuts, chocolate, fish, tomatoes, eggs, fresh berries, and milk. Fresh foods cause hives more often than cooked foods and certain food additives and preservatives may also be to blame.
• Chronic Urticaria and Angioedema – The cause of this type of hive is usually more difficult to identify and tends to last more than six weeks. In some cases, though, the cause may be thyroid disease, hepatitis, infection, or cancer. This type of hives may affect other internal organs like the lungs, muscles, and gastrointestinal tract system and symptoms may include muscle soreness, shortness of breath, vomiting, and diarrhea.
• Physical Urticaria – These hives are caused by direct physical stimulation of the skin such as cold, heat, sun exposure, vibration, pressure, sweating, and exercise. These hives typically happen right where the skin was stimulated and tend to appear within one hour after exposure.
• Dermatographism – This is a common form of physical urticaria where hives form after firmly stroking or scratching the skin. These hives can also occur along with other forms of urticaria.