Buy backpacks, exercise machines, athletic shoes, and more at Fogdog Sports. Prevention Definition Hives is an allergic skin reaction causing localized redness, swelling, and itching .
Hives is an allergic reaction.
The body's immune system is normally responsible for protection from foreign invaders. When it becomes sensitized to normally harmless substances, the resulting reaction is called an allergy. An attack of hives is set off when such a substance, called an allergen, is ingested, inhaled, or otherwise contacted. It interacts with immune cells called mast cells, which reside in the skin, airways, and digestive system. When mast cells encounter an allergen, they release histamine and other chemicals, both locally and into the bloodstream.