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.