Histamine allergic reaction. Animation showing the process that leads to the release of histamine during an allergic reaction. The large spheres are mast cells (mastocytes), a type of white blood cell (leukocyte) found in connective tissue. When first exposed to an allergen, antibodies (Y-shaped, purple) are produced to defend against the allergen. These attach to the surface of the mast cells via receptors (not seen). The second time that person is exposed to the allergen (purple molecular structure), the allergen acts as an antigen and binds to the mast cells, prompting the release of histamine (green spheres). It is the histamine that is responsible for the symptoms of an allergic reaction, such as itching.HD master is flattened onto a black background, as seen in this preview. The original file has a transparent alpha channel and is available as the HD master of clip K003 3737.