Hives occur when something prompts cells to release histamine, a chemical found in the skin. The histamine causes nearby blood vessels to dilate. Fluid leaks out of the dilated vessels and collects under the skin in a raised, flushed, itchy bump called a wheal or hive. Some wheals look like mosquito bites. Wheals often come in groups and may be as small as pencil erasers or as large as 2 to 3 inches across.
Some people know that certain foods or drugs give them hives. For most others, the causes may not be obvious.
"Acute" hives, or hives that are a reaction to a stimulus such as a drug, can last for hours or days. Chronic hives, often of unknown cause, can last for weeks or months.Learn more about hives ›