- granules that contain pr-formed early mediators e.g. histamine, proteases. proteoglycans and chemotactic factors (the black bubbles) - later mediators derived from membrane lipids e.g. leukotrienes, prostaglandins, thromboxanes and prostacyclins. - when an IgE antibody on surface of mast cells become cross linked by an antigen, it triggers the mast cell to degranulate and release large quantities of preformed mediators such as histamine - leukotriene B4 and prostaglandin D2 are produced from membrane lipids