Hives

Hives are welt-like skin rashes that can be as small as a pea and as large and flat as a pancake. They can itch or burn, and be a mild annoyance or cause life-threating symptoms if they develop in the throat. Urticaria (hives) is common, affecting 10% to 20% of people at some point in their lives. Half of those affected continue to have symptoms for more than 6 months after the initial outbreak.

Many substances can trigger hives, including:

• Animal dander (especially cats)
• Insect bites
• Medications
• Pollen
• Shellfish, fish, nuts, eggs, milk, and other foods

Hives may also develop as a result of:

• Emotional stress
• Extreme cold or sun exposure
• Excessive perspiration
• Illness (including lupus, other auto-immune diseases, and leukemia
• Infections such as mononucleosis

What Causes Hives?

Hives occur in response to a complicated chain of events that leads to the release of a chemical called histamine into the skin. Histamine is located in certain white blood cells called mast cells, which are most abundant in the skin around capillaries. If properly triggered, these mast cells release granules of chemicals, the most powerful of which is histamine.

Histamine causes the cells making up the blood vessels to contract allowing fluid to leak out of the blood vessel. Red blood cells are too large to leak out of these "holes". Injecting histamine into the skin causes a triple response of redness, leaking of fluid producing a hive, and the flare or redness around the hive.

Hives are not contagious.