Granules in a mast cell. Coloured transmission electron micrograph (TEM) of a section through granules inside a human mast cell (mastocyte). A mast cell is a type of white blood cell involved in immune and inflammation responses. Here, the mast cell was obtained from the surface mucosa (epithelium) of a stomach ulcer. Four granules are visible. At centre the granule consists of characteristic scrolls that form spirals, made up mainly of proteins. Such granules contain substances like histamine which is released by the mast cell as an allergic or inflammatory response, in this case at the site of a stomach ulcer. Magnification x72,000 at 6x4.5cm size.