When you have an allergic reaction to a substance, histamine and other chemicals are released into your bloodstream, causing itching
, swelling, and other symptoms. The common triggers are medications, foods (shellfish, fish, nuts, eggs, milk, and other foods), pollen and insect bites.
It presents as swelling of the surface of the skin into red- or skin-colored welts (called wheals) which is associated with itching. These welts tend to start suddenly and resolve quickly.