FRANCIS LEROY, BIOCOSMOS / SCIENCE PHOTO LIBRARY FRANCIS LEROY, BIOCOSMOS / SCIENCE PHOTO LIBRARY
Histamine allergic reaction. Diagram showing the process that leads to the release of histamine during an allergic reaction. Cells called mast cells (orange) mediate this response. When an allergy-prone person is first exposed to an allergen, antibodies (Y-shaped) are produced to defend against the allergen and these attach (at left) to the mast cells at receptors yellow, cup-shaped). The second time that person is exposed to the allergen, the allergen acts as an antigen (yellow spheres) and binds to the mast cells, prompting the release of histamine (white). It is the histamine overload that causes the symptoms of an allergic reaction.
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