Although you have great answers, I just wanted to add my two cents regarding the causes of hives.
CausesThe lesions of hives and angioedema are caused by inflammation in the skin. In some cases, hives and angioedema are triggered when certain cells (mast cells) release histamine and other chemicals into your bloodstream and skin.
Allergic reactions to medications or foods can cause acute hives or angioedema. Many allergens have been identified. Examples include:
- Foods. Many foods can cause problems in sensitive people, but shellfish, fish, nuts, eggs and milk are frequent offenders.
- Medications. Almost any medication may cause hives or angioedema; common culprits include antibiotics, aspirin, ibuprofen (Advil, Motrin, others) and blood pressure medications.
- Other allergens. Other substances that can cause hives and angioedema include direct contact with pollen, animal dander, latex and insect stings.
Additional triggers that may produce hives or angioedema include:
- Physical factors. Environmental factors also can result in the release of histamine with subsequent hives or angioedema in some people. Examples of these factors include heat, cold, sunlight, water, pressure on the skin, emotional stress and exercise.
- Dermatographism. The name of this condition literally means "write on the skin." When pressure is applied to the skin or the skin is scratched, raised lines appear on those areas due to histamine-based angioedema that leads to swelling beneath the skin.
In addition to these triggers, hives and angioedema sometimes may occur in response to your body's production of antibodies. Some examples of situations in which this might occur include blood transfusions; immune system disorders, such as lupus or cancer; certain thyroid disorders; and infections, such as hepatitis, or even a cold.
Hereditary angioedema is an inherited form of angioedema and is related to low levels or abnormal functioning of certain blood proteins (C1 inhibitors). These inhibitors play a role in regulating how your immune system functions.
source: Hives