The role of IgE in the natural order is not known, although some scientists suspect that it developed as a defense against infection by parasitic worms. The first time an allergy-prone person is exposed to an allergen, he or she makes large amounts of the corresponding IgE antibody.
Mast cells are plentiful in the lungs, skin, tongue, and linings of the nose and intestinal tract.
When an IgE antibody sitting on a mast cell or basophil encounters its specific allergen, the IgE antibody signals the mast cell or basophil to release the powerful chemicals stored within its granules. These chemicals include histamine, heparin, and substances that activate blood platelets and attract secondary cells such as eosinophils and neutrophils. The activated mast cell or basophil also synthesizes new mediators, including prostaglandins and leukotrienes, on the spot. It is such chemical mediators that cause the symptoms of allergy, including wheezing, sneezing, runny eyes and itching. They can also produce anaphylactic shock, a life-threatening allergic reaction characterized by swelling of body tissues, including the throat, and a sudden fall in blood pressure.