Allergic response. Artwork showing mast cells during an allergic response. Allergens (green structures) from foreign bodies have bound to antibodies (yellow, Y-shaped) on the surface of these white blood cells. This has triggered the release of histamine (tiny red spheres) from the cells' storage granules (purple spheres). An allergic reaction occurs when susceptible people's immune systems become sensitised to a harmless foreign substance. The first exposure to the substance leads to the production of its antibody. Further exposure will then cause an allergic reaction, with histamine release resulting in rash and inflammation.