The red welts of hives happen when mast cells in the bloodstream release the chemical histamine, which causes tiny blood vessels under the skin to leak. The fluid pools within the skin to form spots and large welts. This can happen for a number of reasons, but in many cases a cause is never identified.
Most often, hives are associated with an allergic reaction, which can cause the skin to break out within minutes. Common allergies include:
- foods, especially shellfish, tree nuts, milk, and fruit
- medications and allergy shots
- pets and other animals
- insect bites and stings
Sometimes a breakout of hives has nothing to do with allergies. Other causes include:
- infections, including colds
- anxiety or stress
- exposure to the sun
- exposure to cold, such as from diving into a cold pool
- contact with chemicals
- putting pressure on the skin, such as from sitting too long or carrying a heavy backpack over a shoulder
Hives due to physical causes, such as pressure, cold, or sun exposure, are called physical hives.
It can be hard to figure out what causes chronic urticaria, though it's sometimes linked to an immune system illness, like lupus. Other times, medications, food, insects, or an infection can trigger an outbreak. Often, though, doctors don't know what causes chronic hives.