Sometimes, however, the immune response is hyperactive, and all the symptoms of allergies develop even when the substance causing the response is relatively benign. In the body there are a class of white blood cells, called mast cells, which generally make first contact with allergens. When they do, they release large amounts of histamines a d heparin. These compounds make the walls of tiny blood vessels more permeable, causing the affected tissue to fill with fluid and allowing other immune cells to enter and clear the tissue of the allergen. The body may also be stimulated to secrete mucous, sneeze or cough, all in an attempt to clear it of the potentially harmful substance.
Mast cells are abundant in the skin, eyes, nose, mouth and lungs.
This makes sense, since these are the tissues that are most likely to come into contact with potentially harmful substances. These tissues are also where most allergic reactions occur.