IgE, mast cells, basophils, and eosinophils constitute essential elements in allergic inflammation. Allergen-specific IgE, synthesized in response to allergens in the environment, becomes fixed to FcepsilonRI on the membranes of mast cells and basophils. Aggregation of receptor-bound IgE molecules on re-exposure to specific allergen results in the production of mediators that produce the allergic response. Principal among the cells drawn to sites of mediator release is the eosinophil.