IgE is made in response to an allergen and binds to Fc receptors on mast cells and basophils. The next time the allergen enters the body, it cross-links the Fab portions
of the IgE bound to the mast cell. This triggers the mast cell to degranulate,
that is, release its histamine and other inflammatory mediators. The inflammatory
mediators are now able to bind to receptors on target cells which leads to
dilation of blood vessels, constriction of bronchioles, excessive mucus secretion,
and other symptoms of allergy.