What causes hives?
Hives occur when the body's capillaries and tiny veins get leaky. Fluid escaping from some of these blood vessels gets trapped in parts of the skin and lining membranes of the body, causing the localized swelling in the hives. This leakiness can be caused by classic allergic reactions in which histamine triggers an inflammatory response. It can also be caused by a number of other regulatory systems in the body in response to different types of triggers. Widespread hives are an allergic
reaction to a food, medicine, viral infection, insect bite, or many other possible substances. Often the cause is not found. Hives on just one part of the body (localized) are usually due to skin contact with plants, pollen, food, or pet saliva. Localized hives are not caused by drugs, infections, or swallowed foods. Hives are not contagious.
The most common allergic triggers are drugs (especially antibiotics), foods (especially fish, shellfish, nuts, peanuts, eggs, and food additives), infections (which we'll discuss below), insect bites or stings, inhalants (animal danders, pollens, and molds), and contact allergens (plant substances, skin creams, cat scratches, moth scales, or animal saliva).
There are several known factors that cause histamines to be released, and hives to form. These include: • allergy to medications or substances in the environment • acute or chronic infections • foods • underlying systemic disease such as asthma, a condition that causes inflammation and obstruction of the airways in the lungs • blood products given intravenously, or into the vein • scratching • heat or cold • exposure to sunlight