Itchy body welts, also referred to as hives, appear in response to histamine, a chemical that is released by mast cells, which are its major source, according to Dermatopedia from the Dermatological Society of New Jersey. When the cells release the chemical, itching and welts are experienced almost immediately.
As explained in Dermatopedia, itching and welts generally disappear within 30 minutes once the body metabolizes histamine into an inactive substance. The release of histamine takes place within the first 20 minutes of the ingestion of an allergen. Allergens may be contained in either food or medicine. The avoidance of the allergen prevents hives from reappearing. If a severe reaction is experienced, anaphylaxis may result. Besides hives, sneezing, shock and wheezing are common side effects. Death may occur in some instances of an extreme allergic reaction.