CAROL AND MIKE WERNER / SCIENCE PHOTO LIBRARY CAROL AND MIKE WERNER / SCIENCE PHOTO LIBRARY
Mast cell. This leukocyte (white blood cell) contains the chemical mediators histamine, serotonin and heparin, held in granules (brown spheres) in its cytoplasm. Mast cells are part of the immune system; they release their contents during a localized inflammatory immune response to invading pathogens, and during an allergic reaction, as shown here. The allergen cross bonds with two Immunoglobulin antibodies on the cell surface, triggering the granules to discharge their contents. Histamine and serotonin cause the symptoms of allergies. Heparin is an anticoagulant that may prevent the formation of blood clots between the body's cells.
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