Welts may be caused by a sharp blow or by an allergic reaction, according to MedicineNet. They are raised red areas on the surface of the skin that may or may not be itchy. A welt may be in the form a ridge, a bump or other swelling.
Welts are very often seen in the form of hives, itchy red swellings due to an allergic reaction, says MedlinePlus. They result from histamine and other chemicals being released into the blood and may be triggered by bug bites, particular foods, environmental allergens or medicines. Hives may also appear in response to emotional or physical stress.
Another common condition characterized by welts is angioedema. It's similar to hives, but it affects deeper layers of the skin, according to Mayo Clinic. With angioedema, the welts are thick and firm, and there is usually pain or warmth in the affected area. The same factors that trigger hives can also cause angioedema, as they are both the result of a histamine reaction in the body.
Both hives and angioedema may be associated with underlying medical conditions or illnesses, says Mayo Clinic. Lupus and other autoimmune disorders, lymphoma and certain other cancers, bacterial or viral infections, and some thyroid conditions may provoke hives or angioedema.