Mast cells all have small vesicles called granules, which contain hundreds of chemical mediators that are released when mast cells combine with triggers such as allergy toxins and antigen-IgE interactions. The most common mediator released when mast cells combine with allergenic triggers such as pollen is histamine. Antigens produce IgE after stimulating B cells, and IgE receptors on mast cells bind to and are sensitized by the IgE produced by the antigen; re-exposure of such mast cells to the same antigen results in their release of mediators. The triggers for the release of mediators from mast cells include: allergy toxins such as histamine, leukotrienes, prostaglandins, tumor necrosis factor, neutral proteases, and other substances.