A hives rash, also known as urticaria, is most often associated with an allergic reaction, and in many chronic cases, allergies are responsible. There are other possible causes, and short-lived bouts of hives are often caused by viral or bacterial infection. Heat, cold or constriction from tight clothing or jewelry might irritate the skin and cause a hives rash to form. Hives also might be a symptom of medical conditions such as thyroid, kidney or liver dysfunction, and chronic cases should receive medical attention to determine the cause.

Hives are raised, itchy, red welts on the skin. These welts often appear in clusters, and although they can appear anywhere on the body, they are often observed on the face and extremities. Hives appear quickly, and individual welts might clear quickly while new ones form, making it appear that the hives rash changes shape and location. The welts occur when the capillaries just under the skin leak fluid, causing the region to puff up and swell.

Allergic reactions commonly cause a hives rash. When an allergen is introduced to an allergy sufferer, the body responds by producing histamines. These histamines trigger the capillary leaking that causes the welts to form.

Allergy sufferers are exposed to allergens through ingestion, inhalation or skin contact. Medications such as antibiotics, painkillers and anticonvulsants are all potential allergens. Foods such as nuts and shellfish are closely associated with allergies, but nearly any food can potentially trigger allergic reactions, including a rash. Pollen, mold and animal dander might cause similar results when inhaled. Examples of exposure through skin contact include animal bites and scratches or handling of plants.

Another likely cause of hives is infection. Viral, bacterial and fungal infections might each be responsible for producing a hives rash. Parasites such as pinworms and giardia can also cause hives.

Heat and sweat can cause skin irritation, resulting in hives. Tight clothing and jewelery can constrict the skin and trap sweat, with similar results. Low temperatures might also irritate the skin, and cold is as likely as heat to produce hives.

Many sources of hives cause irritation directly to exposed skin. Hives can appear anywhere on the body, but rashes are especially likely on the exposed skin of the face and extremities. The location of a hives rash might offer a clue about the source.

Frequently, no definitive cause of a rash can be found. Experts agree that viral infection usually is the most likely cause for these cases, especially when the rash spreads to other patients within a short time. Other common causes such as allergens, mold and heat, though less likely, should not be ruled out when attempting to identify the source of the hives rash.