Hives are characterized as itchy red, raised welts (also known as wheals) on the skin's surface that can spread or join together and form larger areas of raised lesions. They are formed by blood plasma leaking out of small blood vessels in the skin. This is caused by the release of a chemical called histamine. Histamine is released from cells called "mast cells" that lie along the blood vessels in the skin. Allergic reactions, chemicals in foods, or medications can cause histamine release. Sometimes it's impossible to find out why hives are forming.
Hives are very common – 10 to 20 percent of the population will have at least one episode in their lifetime. Most episodes of hives disappear quickly, in a few days to a few weeks. Occasionally, a person will continue to have hives for many years. There are many different types and causes of hives including:
Acute Urticaria Hives lasting less than six weeks are called "acute urticaria." With this type of hives, the cause can usually be found. The most common causes are foods, drugs, or infections. Insect bites and internal disease may also be responsible. Other causes can be pressure, cold, or sunlight.
Foods 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. Food additives and preservatives may also cause hives. Hives may appear within minutes or up to two hours after eating, depending on where the food is absorbed in the digestive tract.
Drugs Almost any prescription or over-the-counter medication can cause hives. Some of those drugs include antibiotics, pain medications, sedatives, tranquilizers, and diuretics (fluid pills). Diet supplements, antacids, arthritis medication, vitamins, eye drops and ear drops, laxatives, vaginal douches, or any other non-prescription item can be a potential cause of hives. If you have an attack of hives, it's important to tell your doctor about all of the preparations that you use to assist in finding the cause.
Infections Many infections can cause hives. Colds are a common cause of hives in children.
Chronic Urticaria Hives lasting more than six weeks are called "chronic urticaria." The cause of this type of hives is usually much more difficult to identify than that of acute urticaria. In patients with chronic urticaria, the cause is found in only a small number of patients. Your doctor will need to ask many questions in an attempt to find the possible cause. Since there are no specific tests for hives, testing will depend on your medical history and a thorough examination by your dermatologist.
Physical Urticarias Hives can be caused by sunlight, heat, cold, pressure, vibration, or exercise. Hives due to sunlight are called solar urticaria. This is a rare disorder in which hives form on exposed areas within minutes of sun exposure and fade within one to two hours. Hives due to the cold are more common. These appear when the skin is warmed after exposure to cold. If the cold has affected large areas of the body, large amounts of histamine may be released, which can produce wheezing, flushing, generalized hives, and fainting.
Dermatographic Urticaria Hives that form after firmly stroking or scratching the skin are called "dermatographism." This affects about 5 percent of the population. Most people with this condition are otherwise healthy. These hives can also occur along with other forms of urticaria. They may typically appear in young women and last for months or even years.